Treatment apparatus for surgical correction of defective eyesight, method of generating control data therefore, and method for surgical correction of defective eyesight

ABSTRACT

A treatment method and apparatus for surgical correction of defective-eyesight in an eye of a patient, wherein a laser device is controlled by a control device, said laser device separating corneal tissue by irradiation of laser radiation to isolate a volume located within a cornea, wherein the control device controls the laser device to focus the laser radiation, by providing target points located within the cornea, into the cornea, wherein the control device, when providing the target points, allows for focus position errors which lead to a deviation between the predetermined position and the actual position of the target points when focusing the laser radiation, by pre-offsets depending on the positions of the respective target points to compensate for said focus position errors.

RELATED APPLICATION

The present application claims the benefit of U.S. ProvisionalApplication No. 60/858,201, filed Nov. 10, 2006, which is incorporatedherein in its entirety by reference.

SUMMARY OF THE INVENTION Invention Part A

The invention relates to a planning device for determining control datafor a treatment apparatus for surgical correction of defective eyesightof a patient's eye, said planning device generating control data for thetreatment apparatus comprising a laser device, which separates cornealtissue by irradiation of pulsed laser radiation, said laser radiationbeing focused on target points arranged in a pattern in the cornea.

The invention further relates to a treatment apparatus for surgicalcorrection of defective eyesight of a patient's eye, said apparatuscomprising an interface for supplying measurement data relating toparameters of the eye and defective-eyesight data relating to theeyesight defect to be corrected in the eye, and a laser device, whichseparates corneal tissue by irradiation of pulsed laser radiation, saidlaser radiation being focused on target points arranged in a pattern inthe cornea.

Further, the invention relates to a method of preparing control data fora treatment apparatus for surgical correction of defective eyesight of apatient's eye, which apparatus comprises a laser device separatingcorneal tissue by irradiation of pulsed laser radiation, said laserdevice being operative to focus the laser radiation, according to thecontrol data, on target points arranged in a pattern in the cornea.

Spectacles have long since been the classic way to correct defectiveeyesight of the human eye. Meanwhile, however, increasing use is made ofrefractive surgery which causes a correction of defective eyesight bymodifications of the cornea. All surgical methods aim to selectivelymodify the cornea so as to influence light refraction. Several surgicalmethods are known for this purpose. The most widespread of these is theso-called laser in situ keratomileusis, also abbreviated as LASIK. Inthis method, a corneal lamella (or flap) is first created on one sidefrom the corneal surface and folded aside. This lamella can be createdusing a mechanical microkeratome or a so-called laser keratome asdistributed by Intralase Corp., Irvine, USA, for example.

The latter produces a cut in the cornea by laser radiation. In doing so,several processes take place in the tissue in a time sequence, which areinitiated by the laser radiation. If the power density of the radiationexceeds a threshold value, an optical breakthrough forms, which producesa plasma bubble in the cornea. Said plasma bubble grows due to expandinggases after the optical breakthrough has formed. If the opticalbreakthrough is not maintained, the gas generated in the plasma bubblewill be absorbed by the surrounding material and the bubble willdisappear again. Tissue-separating effects acting without a plasmabubble are also possible. For the sake of simplicity, all such processesare summarized here by the term “optical breakthrough”, i.e. this termis intended to include not only the actual optical breakthrough, butalso the effects resulting therefrom in the cornea.

In order to generate the optical breakthrough, the laser radiation isapplied in a pulsed manner, with the pulse duration being less than 1ps. The required power density of the respective pulse for causing anoptical breakthrough is thus generated only in a tiny spatial region. Inthis respect, U.S. Pat. No. 5,984,916 clearly shows that the spatialregion of the optical breakthrough (of the interaction produced in thiscase) depends strongly on the pulse duration. Thus, high focusing of thelaser beam in combination with the aforementioned short pulses allowsthe optical breakthrough to be placed in the cornea with pinpointaccuracy.

Now, in order to generate the thin flap, a series of opticalbreakthroughs is generated by the laser keratome at predeterminedlocations so as to form a cut surface which detaches the lamella fromthe underlying cornea.

Once the flap has been created and folded aside, the LASIK operationprovides for application of an excimer laser which uses ablation toremove the corneal tissue thus exposed. With volume located in thecornea having been evaporated in this manner, the corneal lamella isfolded back in its initial place. Thus, the LASIK method already in usewhich, when using a laser keratome is also referred to as fs LASIK,creates a lid-shaped corneal lamella, folds it away and ablates thetissue thus exposed using an ablation laser.

The prior art also mentions that the correction of defective eyesight isachieved by isolating a lens-shaped partial volume in the corneal tissueusing the pulsed laser radiation. A corresponding description can befound, for example, in WO 2005/011545 A1. Accordingly, however, devicesare not yet available in the market.

In the LASIK method the corneal tissue exposed by the keratome isablated by an ablation laser such that a desired volume is removedthereby. For this purpose, the laser beam is focused on the exposedcornea in different places so as to remove the material. The materialremoval in the cornea is set by a so-called shot file, which determinesthe number of pulses of the ablation laser radiation and theirrespective coordinates onto which the pulses are emitted. The shot fileis created in the devices after previous measuring of the eye. Due toits different principle of operation, the shot file is not suitable forthe surgical methods and devices presently under scientific examination,which methods and devices isolate a volume in the cornea.

Therefore, it is an object of the invention to provide a planningdevice, an apparatus and/or a method of the above-mentioned typeallowing easy surgical correction of defective eyesight by isolating avolume located in the cornea by means of laser radiation.

According to the invention, this object is achieved by a planning deviceof the above-mentioned type, which uses supplied measurement anddefective-eyesight data to define a volume located within the cornea andwhose removal from the cornea causes the desired correction of defectiveeyesight, said device determining a boundary surface which confines thedefined volume within the cornea and generates for this boundary surfacea control dataset for controlling the laser device, said datasetdetermining a three-dimensional pattern of target points located in theboundary surface and arranged such that the boundary surface is providedas a cut surface upon irradiation of the pulsed laser radiationaccording to the control data set, which cut surface isolates thedefined volume in the cornea and, thus, makes it removable.

According to the invention, the object is also achieved by a treatmentapparatus for surgical correction of defective eyesight of a patient'seye, said apparatus comprising an interface for supplying measurementdata relating to parameters of the eye and defective-eyesight datarelating to the eyesight defect to be corrected in the eye; a laserdevice, which separates corneal tissue by irradiation of pulsed laserradiation, said laser radiation being focused on target points arrangedin a pattern in the cornea, and a planning device of the type describedin the preceding paragraph.

The object is further achieved by a method of the above-mentioned type,which comprises the following steps: determining measurement data onparameters of the eye and defective-eyesight data on eyesight defects tobe corrected in the eye; defining a volume based on the measurement dataand the defective-eyesight data, which volume is located within thecornea and whose removal from the cornea upon operation of the treatmentapparatus results in the desired correction of defective eyesight;determining a boundary surface which confines the defined volume withinthe cornea; determining a three-dimensional pattern of target points inthe cornea, said target points being located in the boundary surface andbeing arranged such that the boundary surface is provided as a cutsurface upon irradiation of the pulsed laser radiation according to thecontrol data, said cut surface isolating the defined volume in thecornea and making it removable in this way; and creating a control dataset which contains three-dimensional patterns for controlling the laserdevice.

According to the invention, a volume is defined first, on the basis ofmeasurement and defective eyesight data of the eye to be corrected,which volume is located within the cornea and whose removal causes thedesired correction of defective eyesight. Removal may be effected, forexample, by a cut opening to the surface of the cornea, which cut makesthe volume accessible and whose creation is also effected by theplanning device or the control data, respectively. For the correction tobe equal, if possible, with respect night vision and day vision, thevolume should cover the pupil of the dark-adapted eye, if possible.Then, boundary surfaces are determined for said volume, which confinethe volume. The boundary surfaces are later provided as cut surfaces bythe treatment apparatus or its laser device, respectively, so that thevolume can be removed, e.g. taken out. For the boundary surfaces, acontrol dataset is determined, defining target points which are locatedin the boundary surface and at which a respective optical breakthroughis to be generated by means of laser radiation so as to form the cutsurface. The target points are a three-dimensional pattern and are alllocated in the previously defined boundary surface. The planning devicemay also be part of the laser device.

As a result of the method or of the activity of the planning device, acontrol dataset is present which enables automatic control of the laserdevice such that the cut surfaces isolating the volume in the cornea maythen be produced automatically. The planning device and the treatmentapparatus equipped with it, respectively, for surgical correction ofdefective eyesight, as well as the method of providing control data fora treatment apparatus preferably creates the control datasetautomatically from the available measurement data and defective-eyesightdata. In advantageous embodiments, the surgeon's assistance is notrequired.

The planning device may be provided as a computer operating under thecontrol of a program. Optionally, the planning device may be part of thetreatment apparatus. Thus, a corresponding computer program productincluding program code which causes the aforementioned method steps isalso a solution of the aforementioned problem.

The control dataset is created on the basis of the determinedmeasurement data and defective-eyesight data of the eye. An independentmeasuring device can acquire these measurement data. Conveniently,however, the treatment apparatus is connected directly to the measuringdevice. Suitable measuring devices include, for example, anautorefractor, a refractometer, a keratometer, an aberrometer, an OCT,or a wavefront measuring device, or any combination of such devices ormeasurement instruments.

The eyesight defect may include hyperopia, myopia, presbyopia,astigmatism, mixed astigmatism (astigmatism in which hyperopia ispresent in one direction and myopia is present in a directionperpendicular to the former), aspherical aberrations and higher-orderaberrations.

A direct data transmission connection of the measuring device to theplanning device, or to the treatment apparatus equipped with saidplanning device, which may be used in one variant, has the advantagethat the use of false measurement and defective-eyesight data is avoidedwith maximum certainty. This applies, in particular, if the transfer ofthe patient from the measuring device(s) to the laser device is effectedby means of a positioning device cooperating with the measuring deviceor with the laser device, respectively, such that the respective devicesrecognize whether the patient is in the respective position formeasurement or for introduction of laser radiation, respectively.Transferring the patient from the measuring device to the laser devicesimultaneously allows the transmission of measurement ordefective-eyesight data to the treatment apparatus.

It is ensured that the planning device always generates the controldataset assigned to the patient and an erroneous use of a wrong controldataset for a patient is virtually impossible.

This aspect is also addressed by a further embodiment of the inventionaccording to which the control dataset is transmitted to the treatmentapparatus and, further, operation of the laser device is preferablyblocked until a valid control dataset is present at the laser device. Inprinciple, a valid control dataset may be any control dataset suitablefor use with the laser device of the treatment apparatus. In addition,however, said validity may also be made subject to the condition thatfurther verifications are performed, e.g. whether a patient's detailsadditionally stored in the control dataset, for example a patientidentification number, correspond to other details which were inputseparately, for example, at the treatment apparatus, as soon as thepatient is in the correct position for operation of the laser device.Transmission may be effected by means of memory chips (e.g. by USB ormemory stick), magnetic storage devices (e.g. diskettes), by radio (e.g.WLAN, UMTS, Bluetooth) or by wire connections (e.g. USB, Firewire,RS232, CAN Bus, ethernet, etc.).

In order to generate the isolated volume, the position of the focus ofthe focused pulsed laser radiation is usually shifted in threedimensions. Therefore, a two-dimensional deflection of the laserradiation, e.g. by scanning mirrors, is usually combined with asimultaneous shift of the focus in the third spatial direction, e.g. bya telescope. Of course, the shift of the focus position is decisive forthe accuracy with which the cut surface isolating the volume can begenerated. It has turned out to be convenient to use a contact glass tobe placed on the eye and fixing the latter. Such a contact glass is alsocommon in the aforementioned laser keratomes used in the fs LASIKmethod. The contact glass usually also functions to impart a known shapeto the anterior surface of the cornea. In the laser keratomes hithertoknown, this shape is a plane, i.e. for operation of the laser keratomethe eye is pressed flat in the region of the cornea. Since this isrelatively inconvenient for the patient, a curved contact glass hasalready been described for approaches isolating a volume in the cornea.Such a contact glass then imparts a curvature to the anterior surface ofthe cornea. Of course, the curvature inevitably results in deformationof the cornea of the eye. Said deformation increases as the curvature ofthe contact surface of the contact glass deviates from the actualcurvature of the cornea of the patient's eye. Therefore, in order to beable to work with a standard contact glass curvature, if possible, it isadvantageous to take the cornea's deformation, which occurs whenapplying a curved contact glass, into consideration in the controldataset, so that the free eye, i.e. the eye not deformed by the contactglass, has the desired boundary surface for the defined volume,regardless of the degree of deformation. Therefore, when generating thecontrol dataset which contains the pattern of the target points, it ispreferred to take any deformation of the eye's cornea intoconsideration, which deformation is present during irradiation of thepulsed laser radiation, in particular a deformation due to theaforementioned contact glass.

This approach allows not only the use, if possible, of a standardizedcontact glass curvature, but at the same time achieves a higher qualityin the correction of defective eyesight. Such consideration of thedeformation is not required, in fact, in the known laser keratomes,because they press the eye flat at the cornea by the contact glass.There, generating the lamella required for the LASIK operation iseffected by simply generating optical breakthroughs in a plane which isparallel to the contact glass.

The generated control dataset can be used directly to control thetreatment apparatus. However, it is convenient to give the treatingphysician a possibility of intervention, allowing him, on the one hand,to verify the control dataset and, on the other hand, to accommodatespecial cases or special wishes. One possible special wish is, forexample, the position of the cut via which the isolated volume is to beremoved from the cornea. This is where ophthalmologists often differ intheir opinions. However, legal reasons of liability often make itdesirable for the physician to have a possibility of intervention.Therefore, it is convenient for the apparatus, as a further embodiment,that the planning device comprises a display device for visual displayof the control dataset and an input device for subsequently altering orinfluencing the control dataset.

Optical systems are generally not perfect. Of course, this also appliesto the focusing of the laser radiation into the cornea. For example,warping of the image field may occur here, as a consequence of whichfocus positions believed to be positioned in a plane are actually notlocated in a plane, but in a curved surface. In the known laserkeratomes, this aspect does not play any role, because producing the cutwhich exposes the lamella has no effect on the optical quality of thecorrection. The actual correction is determined exclusively by thevolume of the exposed cornea evaporated with the ablation laser.Therefore, there is no interest in an error correction, e.g. withrespect to warping of the image field, in the prior art, especiallyconcerning laser keratomes.

Since the volume to be removed is now defined completely by the focusingof the pulsed laser radiation, it is convenient for the planning device,when generating the control dataset, to allow for and, thus, compensatefor any optical focus position errors causing a deviation between thepredetermined position and the actual position of the target points whenfocusing the pulsed laser radiation, by considering a pre-offset whichdepends on the position of the respective target point. Said pre-offsetmay be determined, for example by the planning device accessing acorrection table or correction function which indicates the focusposition error as a function of the position of the respective targetpoint. The correction table or function may be uniformly prescribed forthe respective type of equipment or may be determined individually forthe respective equipment, which is preferred for reasons of precision.

Analog considerations apply to the method according to the invention,wherein optical focus position errors, which cause a deviation betweenthe predetermined position and the actual position of the target pointswhen focusing the pulsed laser radiation, are now offset and compensatedfor by a pre-offset depending on the position of the respective targetpoint, so as to determine the boundary surface or the three-dimensionalpattern of the target points.

The boundary surface isolates the volume if it is provided as a cutsurface after using the pulsed laser radiation. Thus, the boundarysurface automatically has anterior and posterior sections, with theterms “anterior” and “posterior” herein corresponding to standardmedical nomenclature. In principle, it is possible to provide theboundary surface as a free-form surface. However, generating the controldataset is easier if the boundary surface is composed of an anteriorpartial surface and a posterior partial surface. One of said partialsurfaces can then be provided at no constant distance from the surfaceof the cornea. The other one is then inevitably at a constant distancefrom the anterior surface of the cornea. The partial surface located ata constant distance from the anterior surface of the cornea, generallythe anterior partial surface, is thus usually spherical. This appliesexactly if the cornea of the eye is pressed onto a spherical contactglass. The optical correction is then effected by the shape of the otherpartial surface, usually that of the posterior partial surface. Thisconsiderably simplifies the computing effort.

One possibility of indicating the defective-eyesight data consists indetermining the refractive power B_(BR) of spectacles suitable forcorrection of defective eyesight, which have to be arranged at adistance d_(HS) anterior to the corneal vertex so as to achieve thedesired correction of defective eyesight. Determining these parametersis a usual standard in opthalmology and enables the use of wide-spreadand long since introduced measuring devices. In order to generate thecontrol dataset, use is simply made of the defective-eyesight data for aconventional correction by spectacles. It goes without saying that suchdata may also include corrections of astigmatism. A usual formula forthe refractive power B_(BR) of spectacles is, for example, the equation(1) given in the following description of the Figures. It indicates thespherical refraction error Sph as well as the cylindrical refractionerror Cyl wherein, of course, the latter requires that the cylindricalaxis θ be known.

For correction of defective eyesight, a volume is removed from thecornea by the treatment apparatus, i.e. using the control datasetsgenerated in the method according to the invention. The ultimate goal isto alter the curvature of the cornea such that a correction of defectiveeyesight is achieved. A particularly direct, exact and simplecalculation of the curvature of the anterior surface of the cornea to beachieved for correction results from the following equation:R _(CV)*=1/(1/R _(CV))+B _(BR)/((n _(c)−1)(1−d _(HS) ·B _(BR))))+F.

In this equation, R_(CV)* designates the radius of curvature of theanterior surface of the cornea after removal of the volume, R_(CV)designates the radius of curvature of the cornea prior to removing thevolume (this radius being included in the measurement data), n_(c)designates the refractive power of the material of the cornea (generallyabout 1.376), d_(HS) designates the distance at which spectacles havingthe aforementioned refractive power must be located anterior to thecorneal vertex, and F designates a factor which is a measure for theoptical effect of the decrease in the thickness of the cornea on thevisual axis due to the distance of the volume. For a simplifiedcalculation, the factor F can be zero. If a more precise calculation isdesired, F can be calculated as follows:F=(1−1/n _(c))·(d _(C) *−d _(C)),wherein d_(C) or d_(C)*, respectively, designates the thickness of thecornea before and after removal of the volume, respectively. The radiusR_(CV)* is then iteratively calculated by deriving the quantity(d_(C)*−d_(C)) during each iteration step from the difference(R_(CV)*−R_(CV)) and applying the corresponding result, thus obtainedfor the change in thickness, to the calculation of R_(CV)* in the nextiteration step. For example, the iterative calculation is aborted whenonly a small difference for F occurs between two iterative steps andsaid difference is smaller than a limit value.

The method of the invention or the treatment apparatus of the inventioncomprising the planning device operates with particular ease if, asmentioned, the optical correction to be effected by removing said volumeis mainly defined by the curvature of a partial surface which is at anon-constant distance from the anterior surface of the cornea and limitsthe volume. Advantageously, this surface is the posterior partialsurface, because this partial surface then has a radius of curvaturewhich equals the above-mentioned radius of curvature minus the constantdistance between the anterior partial surface and the anterior surfaceof the cornea.

The control dataset provides a file enabling fully automatic surgerywith respect to the control of the treatment apparatus or to acorresponding operation of the treatment apparatus, respectively. Forthis purpose, the control dataset provides the laser device with targetpoints onto which the focused laser beam has to be directed whenemitting laser pulses. Thus, the focus of the focused laser radiation isthen shifted such that follows a path curve over the predeterminedtarget points. In terms of the calculation technique or with respect tothe shifting speed, it is particularly favorable if the path curve is aspiral. In the case of the aforementioned anterior or posterior partialsurface, one spiral is predetermined for each partial surface. With thefocus following a spiral, it is possible to operate the correspondingdeflecting device of the treatment apparatus near its maximum frequency,because e.g. when describing a spiral, two galvanometer scanners can beeach operated near or at their respective maximum frequencies.

When defining the path, it must be generally ensured that the laserpulses are emitted on said path. The target points will then definenodes or sample points of the path. Although the density at which thetarget points predetermine the path may correspond to the density atwhich the points, which receive each a pulse of the laser radiation, arearranged on the path, this is not strictly required. On the contrary, itis even preferred that the target points represent only a subset ofthose points onto which laser pulses are emitted. On the one hand, thecontrol dataset will then be drastically reduced in terms of its datavolume; on the other hand, the calculating effort will be reduced in allthose steps where a non-functional description of the path curve in theboundary surface shall or can be worked with, but where the target pointcoordinates have to be processed individually. An example of this is theaforementioned correction with respect to the curvature of the imagefield.

Therefore, it is preferred for the apparatus if the laser device shiftsthe focused laser radiation along a path over a pattern of targetpoints, with pulses of the pulsed laser radiation being emitted into thecornea also at points located on the path and between the target points.

This applies in analogy to the method according to the invention, namelythat the control dataset is provided for a laser device which shifts thefocused laser radiation along a path over the pattern of target points,wherein the control dataset is generated such that the target points inthe pattern represent only a subset of the points onto which the laserdevice emits the pulsed laser radiation. The control dataset is thusadapted to the shifting speeds of the laser device which are achievable.

As a result, the frequency of the sample points (or nodes) predeterminedor applied in the laser device for shifting the focus position differsfrom that which occurs when generating the laser pulses. Of course, thecontrol dataset per se usually does not contain any indication relatingto the frequency, even if this is possible. Due to the maximum shiftingspeed or the highest signal frequencies applied when shifting the focusof the laser radiation, the definition of the target points naturallycorresponds to a path speed or a shifting speed in the respectivecoordinates used for description. In the preferred embodiment, thespacing between the target points in combination with the path speed andthe laser pulse frequency which can be realized by the laser device nowhas the effect that laser pulses are also emitted automatically atpoints in time where the focus is still shifted from one target point tothe next. This approach has the advantage that, during operation of thetreatment apparatus, the target points are/are to be predetermined witha frequency of less than the frequency at which the pulses of the pulsedlaser radiation are/are to be emitted into the cornea by the laserdevice.

It goes without saying that predetermining target points in the controldataset does not mean that the focus has to rest, i.e. that a shiftingspeed equaling zero has to be present at these target points when thelaser pulse is emitted onto the target point. In the sense of quicklyproducing as the cut surface as the determined boundary surface of thedefined volume, it is advantageous if the synchronization of the shiftin focus position and the emission of the laser pulses is achieved suchthat a laser pulse, although emitted under continuous deflection of thefocus, still enters the cornea at the target point. Thus, the pulsedlaser radiation is applied while continuously shifting the focusposition, e.g. in moving scanning mirrors. This feature causes asystematic difference over known shot files for ablation lasers, whereina shot of the ablation laser is not emitted until the deflection of thelaser beam steadily targets a determined point.

Invention Part B

The invention relates to a treatment apparatus for surgical correctionof defective eyesight in the eye of a patient, which apparatus comprisesa laser device controlled by a control device, said laser deviceseparating corneal tissue by irradiation of laser radiation so as toisolate a volume located within the cornea, wherein the control devicecontrols the laser device for focusing the laser radiation into thecornea by providing target points located in the cornea.

The invention further relates to a method of generating control data fora laser device of a treatment apparatus for surgical correction ofdefective eyesight in the eye of a patient, said laser device separatingcorneal tissue by irradiation of laser radiation so as to isolate avolume located within the cornea, wherein during operation of thetreatment apparatus the control data provide the laser device withtarget points for the focused laser radiation, said target points beinglocated within the cornea.

Finally, the invention further relates to a method of surgicalcorrection of defective eyesight in the eye of a patient, wherein avolume is isolated in the cornea and, for this purpose, laser radiationis focused on target points arranged in a pattern within the cornea, soas to separate corneal tissue.

Spectacles are the classic means of correction defective eyesight of thehuman eye. Meanwhile, however, increasing use is made of refractivesurgery which causes a correction of defective eyesight by modifying thecornea. Several surgical methods aim to selectively modify the cornea soas to influence light refraction. Different surgical methods are knownfor this purpose. The most widespread of these is presently theso-called laser in situ keratomileusis, also abbreviated as LASIK. Inthis method, a corneal lamella (or flap) is first created on one side atthe corneal surface and folded aside. Detachment of this lamella can beeffected using a mechanical microkeratome or a so-called laser keratomeas distributed by Intralase Corp., Irvine, USA, for example. Once thelamella has been created and folded aside, the LASIK operation uses anexcimer laser which, by ablation, removes corneal tissue thus exposed.Once a volume of the cornea has been evaporated in this manner, thecorneal lamella is folded back to its initial place.

The application of a laser keratome for creating the flap isadvantageous, because it will decrease the risk of infection andincrease the quality of the cut. In particular, the flap can be producedwith a much more constant thickness. Also, the cut is potentiallysmoother, thus reducing later optical interferences by this boundarysurface which remains even after surgery.

When generating a cut surface in the cornea by laser radiation, severalprocesses take place in sequence, initiated by the pulsed laserradiation. If the power density of the radiation exceeds a thresholdvalue during a pulse, an optical breakthrough forms, which produces e.g.a plasma bubble in the cornea. Said plasma bubble grows due to expandinggases after the optical breakthrough has formed. If the opticalbreakthrough is not maintained, the gas generated in the plasma bubblewill be absorbed by the surrounding tissue, and the bubble willdisappear again. Tissue-separating effects acting without a plasmabubble are also possible. For the sake of simplicity, all such processesare summarized here by the term “optical breakthrough”, i.e. this termis intended to include not only the actual optical breakthrough, butalso the effects resulting therefrom in the cornea.

In order to separate tissue, the laser radiation is applied in pulsedform, with the pulse duration usually being less than 1 ps. The requiredpower density of the respective pulse for causing the opticalbreakthrough is thus generated in a tiny spatial region. In thisrespect, U.S. Pat. No. 5,984,916 clearly shows that the spatial regionof the optical breakthrough (of the interaction produced in this case)depends strongly on the pulse duration. Thus, high focusing of the laserbeam in combination with the aforementioned short pulses allows theoptical breakthrough to be placed in the cornea with pinpoint accuracy.

To produce a cut, a series of optical breakthroughs are generated atpredetermined locations such that a cut surface is generated thereby. Inthe aforementioned laser keratome, the cut surface forms the lamella tobe folded aside prior to the use of laser ablation.

Of course, the precision with which the cut surface is generated isdecisive for the optical correction in the end. This applies, inparticular, to advanced laser surgery correction methods for defectiveeyesight, wherein a volume located within the cornea is isolated by athree-dimensional cut surface to make it removable. In contrast to thelaser keratome, the position of the cut surface is then directlyrelevant for optical correction. On the other hand, in the conventionalLASIK method, only the precision with which the laser ablation iseffected is important for the quality of the optical correction, whichcan be recognized already by the fact that the generation of the corneallamella is also possible with a mechanical knife working in acomparatively coarse manner and is or has been practiced also in a greatmultiplicity of surgeries.

Therefore, it is an object of the invention to provide a treatmentapparatus or a method of the aforementioned type such that cut surfacescan be produced with great precision.

The invention acknowledges the fact that optical systems are usually notperfect. An error in focusing the laser radiation in the cornea has aneffect on the production of cut surfaces. This applies, in particular,to errors in the focus position due to which focus positions believed tobe positioned in a plane are actually not located in a plane, but in acurved surface. In the known laser keratomes, this aspect does not playany role, because producing the cut which exposes the lamella has nomeaning for the optical quality of the correction. However, when thevolume to be isolated and to be removed is defined completely by thefocusing of the pulsed laser radiation, the method according to theinvention provides that focus position errors causing a deviationbetween the predetermined position and the actual position of the targetpoints during focusing of the laser radiation, which is preferablyapplied in pulsed form, are allowed for by a corresponding pre-offset inthe opposite direction.

Said pre-offset causes a pre-distortion of the arrangement of targetpoints such that, in combination with the focus position error, thetarget points are re-located at the position desired for the use of thetreatment apparatus.

The pre-offset may be conveniently determined by a correction table orcorrection function which indicates the focus position error as afunction of the position of the respective target point. This table orfunction may be uniformly prescribed for the respective type ofequipment or, which is preferred for reasons of precision, may beindividually determined for the respective equipment, for example by themanufacturer of said equipment or upon installation of the equipment atthe user's site.

The optics used to focus the laser radiation can be provided such, withan acceptable effort, that the focus position error is substantially anaxial focus position error and rotation-symmetric to the optical axis.For this variant, it is then preferred that each pre-offset shifts therespective target point parallel to the optical axis and that thecorrection table or function depend only on an axial coordinate and aradial coordinate when the target points are described in cylindricalcoordinates. For convenience, the origin of the cylindrical coordinatemay be placed at the point where the optical axis passes through theanterior surface of the cornea or through a vertex of any contact glassused.

An analytic definition of the set of target points can be selected whendetermining the target points. This is particularly easy if a path isdefined along which the focus of the laser radiation is to be shifted soas to form the desired cut surface which will isolate the volume. Thetarget points merely need to be selected from among the points of thepath. Since the correction with respect to the focus position error willusually not be present as a transcendent, analytic or even linearfunction, and in most cases only an interpolation by polynomials orsplines will be possible at best, it is preferred to select the targetpoints from the path prior to the correction. This approach has theadvantage that a simple, low computational effort is necessary todetermine the target points.

Simplicity is increased further if the path is determined by one or morefunctional equation (s) such that the evaluation of said functionalequation (s) yields the target points at different nodes. Until then, atranscendent description of the beam deflection is possible, whichnaturally incurs a very strongly reduced calculating effort than if alarge quantity of target points is managed. The actual targetcoordinates are present only after evaluation of the functional equation(s) at the nodes. They will then represent control data which will beapplied during operation of the treatment apparatus. Of course, thesecontrol data can be still further processed so as to allow fordevice-specific adaptations, e.g. determination of corresponding voltagelevels for the coordinates, the previously determined amplitude or phasecharacteristics of galvanometer mirrors, etc.

As already mentioned, the position of the focus of the generally pulsedlaser radiation is three-dimensionally shifted in order to produce a cutsurface. Therefore, a two-dimensional deflection of the laser radiation,e.g. by galvanometer mirrors, is usually combined with a simultaneousfocus shift in the third spatial direction, e.g. by a telescope. Ofcourse, the adjustment of the focus position is decisive for theaccuracy with which the cut surface can be generated by the targetpoints. It has turned out to be convenient for this purpose to use acontact glass to be placed on the eye and fixing the latter. Such acontact glass is also common in the aforementioned laser keratomes. Thecontact glass usually also functions to impart a known shape to theanterior surface of the cornea. In the known laser keratomes, the shapeis a plane, i.e. the eye is pressed flat in the region of the cornea.Since this is relatively inconvenient for the patient, a curved contactglass comprising a curved contact surface facing the eye has alreadybeen described for approaches isolating a volume in the cornea by lasersurgery. Such a contact glass then imparts a curvature to the anteriorsurface of the cornea. Of course, the curvature inevitably results indeformation of the cornea of the eye. Said deformation increases as thecurvature of the contact surface deviates from the actual curvature ofthe cornea of the patient's eye increases. Therefore, in order to beable to work with a standard contact surface curvature, it isadvantageous to take into consideration the deformation of the corneawhich occurs when applying a curved contact glass, so that the targetpoints for the free eye, i.e. the eye not deformed by the contact glass,are ultimately located at the desired positions, regardless of thedegree of deformation, and thereby a desired boundary surface isultimately generated for the volume to be removed.

On the one hand, the deformation of the cornea can be taken into accounton the basis of the target points, i.e. when the individual coordinatesof the target points have been determined. On the other hand, thedeformation can also be taken into account by means of a correspondingtransformation of the path along which the focus is to be shifted. It ispreferred for the treatment apparatus that the control device, whendetermining the path or the target points, take a deformation of thecornea into consideration, which deformation occurs during irradiationof the pulsed laser radiation, in particular due to a contact glass.Analog features apply to the method according to the invention.

It is particularly favorable in terms of the calculating load,especially for a path given by transcendent functional equations, tofirst determine the path with respect to the non-deformed eye and thento modify said path so as to compensate for the deformation of thecornea of the eye. In a final step, the path function(s) are evaluatedat certain nodes, so as to determine the coordinates for the targetpoints, which are then corrected in a final step, as described, withrespect to focus position errors.

Particularly simple computational treatment is obtained if the targetpoints are arranged along paths which extend in a spiral shape. Forexample, the cut surface to be produced, which isolates the volume, canbe divided into two partial surfaces or sub-surfaces, an anteriorpartial surface and a posterior partial surface. The sub-surfaces can berespectively formed by shifting the focus along a spiral-shaped path.

The method according to the invention for preparing the control data canalso be effected without human assistance. In particular, it may beperformed by a computer which determines from corresponding data, e.g.from the functionally defined path curves, the control data by selectingthe nodes matching the shifting speed of the laser device whichrepresents the target system of the control data. In particular,determining the control data does not require any assistance from aphysician, because said determination of the control data does not yetinvolve any therapeutic intervention. Such intervention will take placeonly upon application of the previously determined control data.

Invention Part C

The invention relates to a treatment apparatus for surgical correctionof defective eyesight in the eye of a patient, said apparatus comprisinga laser device which is controlled by a control device and whichseparates corneal tissue by irradiation of laser radiation, wherein thecontrol device controls the laser device to focus the laser radiationinto the cornea at target points arranged in a pattern within the corneaand selects the pattern such that a volume is thereby isolated in thecornea, with the removal of said volume from the cornea causing thedesired correction of defective eyesight.

The invention further relates to a method of generating control data fora laser device of a treatment apparatus for surgical correction ofdefective eyesight in the eye of a patient, which laser device separatescorneal tissue by irradiation of focused laser radiation, wherein thecontrol data provide the laser device with target points for the focusedlaser radiation during operation of the treatment apparatus, whichtarget points are arranged in a pattern within the cornea such that avolume is thereby isolated in the cornea, with the removal of saidvolume from the cornea causing the desired correction of defectiveeyesight.

Finally, the invention relates to a method for surgical correction ofdefective eyesight in an eye of a patient, wherein, in order to separatecorneal tissue, laser radiation is focused onto target points arrangedin a pattern within the cornea and, thereby, a volume is isolated in thecornea, with the removal of said volume causing the desired correctionof defective eyesight.

Spectacles are the classic means of correction defective eyesight of thehuman eye. Meanwhile, however, increasing use is made of refractivesurgery which causes a correction of defective eyesight by modifying thecornea. Said surgical methods aim to selectively modify the cornea so asto influence light refraction. Several surgical methods are known forthis purpose. The most widespread of these is presently the so-calledlaser in situ keratomileusis, also abbreviated as LASIK. In this method,a corneal lamella (or flap) is first created on one side at the cornealsurface and folded aside. Detachment of this lamella can be effectedusing a mechanical microkeratome or a so-called laser keratome asdistributed by Intralase Corp., Irvine, USA, for example. Once thelamella has been created and folded aside, the LASIK operation uses anexcimer laser which, by ablation, removes corneal tissue thus exposed.Once a volume of the cornea has been evaporated in this manner, thecorneal lamella is folded back to its initial place.

The application of a laser keratome for creating the flap isadvantageous, because it will decrease the risk of infection andincrease the quality of the cut. In particular, the flap can be producedwith a much more constant thickness. Also, the cut is potentiallysmoother, thus reducing later optical interferences by this boundarysurface which remains even after surgery.

When generating a cut surface in the cornea by laser radiation, severalprocesses take place in sequence, initiated by the pulsed laserradiation. If the power density of the radiation exceeds a thresholdvalue during a pulse, an optical breakthrough forms, which produces e.g.a plasma bubble in the cornea. Said plasma bubble grows due to expandinggases after the optical breakthrough has formed. If the opticalbreakthrough is not maintained, the gas generated in the plasma bubblewill be absorbed by the surrounding tissue, and the bubble willdisappear again. Tissue-separating effects acting without a plasmabubble are also possible. For the sake of simplicity, all such processesare summarized here by the term “optical breakthrough”, i.e. this termis intended to include not only the actual optical breakthrough, butalso the effects resulting therefrom in the cornea.

In order to separate tissue, the laser radiation is applied in pulsedform, with the pulse duration usually being less than 1 ps. The requiredpower density of the respective pulse for causing the opticalbreakthrough is thus generated in a tiny spatial region. In thisrespect, U.S. Pat. No. 5,984,916 clearly shows that the spatial regionof the optical breakthrough (of the interaction produced in this case)depends strongly on the pulse duration. Thus, high focusing of the laserbeam in combination with the aforementioned short pulses allows theoptical breakthrough to be placed in the cornea with pinpoint accuracy.

To produce a cut, a series of optical breakthroughs are generated atpredetermined locations such that a cut surface is generated thereby. Inthe aforementioned laser keratome, the cut surface forms the lamella tobe folded aside prior to the use of laser ablation.

In the conventional LASIK method, exposed corneal tissue is evaporated,which is also referred to as “abrading” the cornea by means of laserradiation. The volume removal required for a correction of defectiveeyesight is then performed for each surface element of the exposedcornea by the number of laser pulses and their energy. Therefore, in theLASIK method a so-called shot file is provided for the ablation laser,which file predetermines for various points on the cornea of the eye howoften laser beam pulses are to be directed on defined points on thecornea and with what energy. In doing so, the volume removal washeuristically determined, not least because it depends largely on theablation effect of the laser beam and, thus, on the wavelength, thefluence, etc. of the radiation employed. The condition of the eye'scornea also plays a role; here, the moisture content of the eye's corneais to be mentioned.

Now, values gained by experience, which are suitable for abrading thecornea by means of ablation laser radiation, can not be used forimproved methods of refractive eye surgery, wherein the volume to beremoved from the cornea is not removed by ablation of exposed cornealtissue, but is isolated by a three-dimensional cut surface in the corneaand is thus made removable.

Therefore, it is an object of the present invention to provide atreatment apparatus or method of the above-mentioned type such that thecut surface can be precisely defined.

According to the invention, this object is achieved by a treatmentapparatus of the above-mentioned type, wherein the cornea reduced bysaid volume has a radius of curvature R_(CV)* which satisfies thefollowing equation:R _(CV)*=1/((1/R _(CV))+B _(BR)/((n _(c)−1)(1−d _(HS) ·B _(BR))))+F,wherein R_(CV) is the radius of curvature of the cornea prior to removalof said volume, n_(c) is the refractive power of the material of thecornea, F is a correction factor, B_(BR) is the refractive power ofspectacles suitable to effect the correction of defective eyesight, andd_(HS) is the distance at which said spectacles having the refractivepower B_(BR) would have to be located anterior of the corneal vertex inorder to achieve the desired correction of defective eyesight by meansof the spectacles.

According to the invention, this object is further achieved by a methodfor generating control data for a laser device of the above-mentionedtype, wherein in the method for generating control data for a laserdevice of a treatment apparatus for surgical correction of defectiveeyesight in a patient's eye, which laser device separates corneal tissueby irradiation of focused laser radiation, control data provide thelaser device with target points for the focused laser radiation duringoperation of the treatment apparatus, which target points are arrangedin a pattern within the cornea such that a volume is thereby isolated inthe cornea, with removal of said volume from the cornea causing thedesired correction of defective eyesight.

Finally, the object is also achieved by a method for surgical correctionof defective eyesight as mentioned above, wherein the cornea reduced bysaid volume assumes a radius of curvature R_(CV)* which satisfies thefollowing equation:R _(CV)*=1/(1/R _(CV))+B _(BR)/((n _(c)−1)(1−d _(HS) ·B _(BR))))+F,wherein R_(CV) is the radius of curvature of the cornea prior to removalof said volume, n_(c) is the refractive power of the material of thecornea, F is a corrective factor, B_(BR) is the refractive power ofspectacles suitable to effect the correction of defective eyesight, andd_(HS) is the distance at which the spectacles having the refractivepower B_(BR) would have to be located anterior of the corneal vertex inorder to achieve the desired correction of defective eyesight by meansof the spectacles.

Thus, the invention provides a control value or a calculation parameter,on the basis of which the volume to be removed and, thus, the cutsurface isolating said volume in the cornea can be calculated as exactlyas possible. The invention defines an equation for the radius ofcurvature which the cornea has after removal of the volume isolated bythe treatment apparatus or the method, respectively. This equationallows the volume to be calculated in an analytically exact manner. Theheuristic approach, as applied when abrading the cornea exposed in theLASIK process, is replaced by an analytic description of the anteriorsurface of the cornea, as it has to be present after correction, whichenables precise calculation of the volume to be removed in the improvedophthalmic surgical method.

The description of the curvature of the anterior corneal surface aftercorrection is based on defective-eyesight data indicating the refractivepower B_(BR) of spectacles suitable for the correction of defectiveeyesight, which spectacles have to be located at a distance d_(HS)anterior to the corneal vertex in order to achieve the desiredcorrection of defective eyesight. Determining these parameters is ausual standard in opthalmology and enables the use of wide-spread andlong since introduced measuring devices. Thus, when calculating thevolume to be isolated in the cornea, defective-eyesight data for aconventional correction by spectacles are to be used.

Of course, such data can also include astigmatism corrections or evencorrections of higher orders of aberration. A usual formula for therefractive power B_(BR) of spectacles is, for example, the equation (1)given in the following description of the Figures. It indicates thespherical refraction error Sph as well as the cylindrical refractionerror Cyl and, of course, the latter requires knowledge of itscylindrical axis θ.

The correction factor F is a measure of the optical effect of thereduction in the thickness of the eye's cornea on the visual axis due toremoval of said volume. In a simplified calculation, the factor F can beset to zero. If a more precise calculation is desired, F can becalculated as follows:F=(1−1/n _(c))·(d _(C) *−d _(C)),wherein d_(C) and d_(C)* respectively refer to the thickness of thecornea before and after removal of said volume, and the radius R_(CV)*is iteratively calculated, by deriving a change in thickness(d_(C)*−d_(C)) from the difference (R_(CV)*−R_(CV)) in each iterationstep and applying the corresponding result, thus obtained for the changein thickness, to the calculation of R_(CV)* in the next iteration step.For example, the iterative calculation of F is aborted when only adifference for F occurs between two iterative steps which is smallerthan a limit value.

Taking into consideration a cylindrical eyesight defect, the radiuswhich the cornea has after reduction by said volume, naturally is afunction of the cylindrical angle, i.e. of an angle perpendicular to thevisual axis, as is common in opthalmology to describe an astigmaticeyesight defect. Of course, the same applies for the refractive power ofspectacles on which the equation for the radius is based.

Now, according to the invention, the volume is determined ordeterminable such that the cornea has the defined radius of curvatureafter removal of said volume. A definition of the volume which isparticularly easy to calculate and, above all, also easy to realizelimits the volume by a boundary surface which is divided into ananterior and a posterior partial surface, with the anterior partialsurface being located at a constant distance d_(F) from the anteriorcorneal surface. The terms “anterior” and “posterior” correspond tostandard medical nomenclature.

Due to the anterior partial surface located at a constant distance fromthe corneal surface, this partial surface is particularly easy togenerate. Of course, the posterior partial surface will then inevitablyhave no constant distance from the anterior corneal surface. Opticalcorrection is effected by the shape given to the posterior partialsurface. This approach considerably reduces the calculating effort,because a spherical partial surface (the anterior partial surface) isparticularly easy to calculate, and the calculating effort isconcentrated on determining the posterior partial surface.

It has turned out, in a surprising manner, that such an approach enablesa simple analytic description of the posterior partial surface at thesame time. This is because said surface has a curvature which can beidentical, except for an additive constant, with the curvature of theanterior corneal surface after removal of said volume. Said constantincorporates the distance which the anterior partial surface isposterior of the anterior corneal surface.

In particular, this embodiment allows the posterior partial surface tobe described with particular ease by cylindrical coordinates whoseorigin is located at the point where the visual axis intersects theanterior corneal surface, namely by the equation:z _(L)(r,φ)=R _(L)(φ)−(R _(L) ²(φ)−r ²)^(1/2) +d _(L) +d _(F)wherein d_(L) sets a minimum thickness of the volume to be removed.

The minimum thickness d_(L), which is optional, assists the subsequentremoval of the isolated volume, because said volume then has a certainminimum thickness (e.g. at its periphery), namely the value d_(L). Atthe same time, it can thus be ensured that the volume fully covers thepupil of the eye, preferably even when the eye is dark-adapted. Thisensures that the optical correction is optimal for all visual conditionsencountered by the patient.

The method according to the invention for preparing the control data canbe effected without human assistance. In particular, it may be performedby a computer which determines from corresponding data, e.g. from thefunctionally defined path curves, the control data by effecting theselection of the sample points or nodes in a manner matching theshifting speed of the laser device representing the target system forapplication of the control data. In particular, determining the controldata does not require any assistance from a physician, because saiddetermination of the control data does not yet involve a therapeuticintervention. Such intervention will take place only upon application ofthe previously determined control data.

Invention Part D

The invention relates to a treatment apparatus for surgical correctionof defective eyesight in a patient's eye, said apparatus comprising alaser device which separates corneal tissue by irradiation of pulsedlaser radiation, said laser radiation being focused on target pointsarranged in a pattern within the cornea.

The invention further relates to a method for generating control datafor a laser device of a treatment apparatus for surgical correction ofdefective eyesight in a patient's eye, which laser device separatescorneal tissue by irradiation of focused, pulsed laser radiation havinga specific pulse frequency, wherein the laser device is provided withtarget points for the pulsed laser radiation which are arranged in apattern within the cornea.

Further, the invention relates to a method for surgical correction ofdefective eyesight in a patient's eye, wherein pulsed laser radiation isfocused on target points, arranged in a pattern within the cornea, so asto separate corneal tissue.

Spectacles are the classic means of correction defective eyesight of thehuman eye. Meanwhile, however, increasing use is made of refractivesurgery which causes a correction of defective eyesight by modifying thecornea. Said surgical methods aim to selectively modify the cornea so asto influence light refraction. Several surgical methods are known forthis purpose. The most widespread of these is presently the so-calledlaser in situ keratomileusis, also abbreviated as LASIK. In this method,a corneal lamella (or flap) is first created on one side at the cornealsurface and folded aside. Detachment of this lamella can be effectedusing a mechanical microkeratome or a so-called laser keratome asdistributed by Intralase Corp., Irvine, USA, for example. Once thelamella has been created and folded aside, the LASIK operation uses anexcimer laser which, by ablation, removes corneal tissue thus exposed.Once a volume of the cornea has been evaporated in this manner, thecorneal lamella is folded back to its initial place.

The application of a laser keratome for creating the flap isadvantageous, because it will decrease the risk of infection andincrease the quality of the cut. In particular, the flap can be producedwith a much more constant thickness. Also, the cut is potentiallysmoother, thus reducing later optical interferences by this boundarysurface which remains even after surgery.

When generating a cut surface in the cornea by laser radiation, severalprocesses take place in sequence, initiated by the pulsed laserradiation. If the power density of the radiation exceeds a thresholdvalue during a pulse, an optical breakthrough forms, which produces e.g.a plasma bubble in the cornea. Said plasma bubble grows due to expandinggases after the optical breakthrough has formed. If the opticalbreakthrough is not maintained, the gas generated in the plasma bubblewill be absorbed by the surrounding tissue, and the bubble willdisappear again. Tissue-separating effects acting without a plasmabubble are also possible. For the sake of simplicity, all such processesare summarized here by the term “optical breakthrough”, i.e. this termis intended to include not only the actual optical breakthrough, butalso the effects resulting therefrom in the cornea.

In order to separate tissue, the laser radiation is applied in pulsedform, with the pulse duration usually being less than 1 ps. The requiredpower density of the respective pulse for causing the opticalbreakthrough is thus generated in a tiny spatial region. In thisrespect, U.S. Pat. No. 5,984,916 clearly shows that the spatial regionof the optical breakthrough (of the interaction produced in this case)depends strongly on the pulse duration. Thus, high focusing of the laserbeam in combination with the aforementioned short pulses allows theoptical breakthrough to be placed in the cornea with pinpoint accuracy.

To produce a cut, a series of optical breakthroughs are generated atpredetermined locations such that a cut surface is generated thereby. Inthe aforementioned laser keratome, the cut surface forms the lamella tobe folded aside prior to the use of laser ablation.

Obviously, the cut surface is to be generated as quickly as possible, ofcourse. Therefore, a pulse frequency of the laser which is as high aspossible is desired. Naturally, this also increases the control effortaccordingly.

This applies, in particular, if advanced applications of refractivesurgery are desired, wherein the volume to be removed from the cornea isnot removed by ablation of exposed corneal tissue, but by creating athree-dimensional cut surface which encloses the volume to be removed.Such applications involve not only more complex cut shapes, but alsorequire processing of a considerably larger cut surface.

Therefore, it is the object of the present invention to provide anapparatus of the above-mentioned type or a corresponding method,respectively, by which a cut surface can be produced with little effortand as quickly as possible.

According to the invention, this object is achieved by a treatmentapparatus of the above-mentioned type, wherein the laser device shiftsthe focused laser radiation along a path via the pattern of targetpoints and emits pulses of the pulsed laser radiation into the corneaeven at points which are located on the path between the target points.

The object is further achieved by a method of generating control data ofthe above-mentioned type, wherein the control data define the targetpoints as points of a path along which the focus of the laser radiationis to be shifted during the intended operation of the treatmentapparatus, with the target points being spaced apart on the path suchthat, during operation of the treatment apparatus, pulses of the pulsedlaser radiation are also emitted into the cornea at points located onthe path between the target points, due to the focus shifting speed andthe pulse frequency of the laser device.

Finally, the object is also achieved by a method for surgical correctionof defective eyesight as mentioned above, wherein the focused laserradiation is shifted along a path via the pattern of the target pointsand pulses of the pulsed laser radiation are emitted into the corneaalso at points located on the path between the target points.

Thus, the invention no longer provides one target coordinate exactly foreach laser pulse. Instead, target points are provided at a greaterdistance than that at which the laser pulses are located in the cornea.Thus, laser pulses which were emitted into the cornea while shifting thefocus position from one target point to the next are located between thelaser pulses for which a target point was provided.

This concept has the advantage that the control requirements for thefocus shifting device, which usually performs a three-dimensional focusshift (because the cut surfaces can be assumed to be three-dimensional),are drastically reduced.

If a continuous beam deflection is provided for at the same time, a veryquick production of cut surfaces is achieved in addition. In contrast toconventional excimer lasers, there is no need to wait, for each laserpulse, until the focus shifting device has been set to the coordinatesof the currently next target point. Rather, a continuously adjustingfocus shifting device can be employed. This is particularly advantageousif the cut surfaces are constituted by path curves having a spiralshape. If conventional galvanometer scanning mirrors are then used inthe focus shifting device, these can be controlled with oscillationsnear the scanner's limit frequency, thereby increasing the deflectionspeed to the maximum technically feasible level.

However, in addition to a simpler control and quicker constitution ofthe cut surface, respectively, the invention also results in surprisingsimplifications when generating the target coordinates.

In this respect, it is particularly advantageous if the paths on whichthe later target points will be located are described, if possible, inthe form of functional equations. Having finally defined the pathcurves, the target points will then be generated by suitable evaluationsof the functional equation at nodes which are spaced apart from eachother by a selectable amount.

Isolating a volume by a three-dimensional cut surface and making itremovable naturally requires datasets which are considerably larger thanthe datasets of conventional shot files of excimer lasers. On the onehand, this is due to the fact that the third space coordinate, namelythe z coordinate, now has to be specified, too. In contrast thereto, inablation by means of laser radiation, there is always an interactiontaking place only on the material surface, so that no z coordinate hasto be specified. As a consequence, three-dimensional specifications arerequired now instead of two-dimensional coordinate sets, thus inevitablyincreasing the size of the control datasets. Moreover, it is nowrequired to process comparatively larger surfaces than those that wererequired in conventional laser ablation. This increases the size of thecontrol datasets additionally.

The inventive approach of no longer providing an explicit targetcoordinate for each point where a laser beam pulse is to be introducedinto the cornea helps to drastically reduce the dataset. For example,for a pulse frequency of the pulsed laser radiation of between 50 and300 kHz, the node frequency may be ⅕ to 1/50 of the laser pulsefrequency. Of course, the volume of the dataset is also reduced by thecorresponding factor. This drastically reduces the transmission overheadas well as the calculating effort for determining the target points andgenerating the control dataset.

This advantage is particularly noticeable if the path curves, on whichare located the points onto which the laser radiation pulses areemitted, are treated, as long as possible, in the form of functionaldescriptions. In the case of an adroit selection of algorithms, anexplicit calculation of target coordinates, i.e. the functionalevaluation of the path curve function, is required only if a focusposition error has to be corrected, because it usually cannot bedescribed in analytic, functional terms and usually can be described, atbest, by interpolation methods, such as polynomial interpolation orspline interpolation. Therefore, it is preferred to functionallydescribe the path curves and to effect an evaluation of the functionalequations for calculation of the target point coordinates only in caseof and, if possible, directly before a compensation of a focus positionerror.

Of course, when providing the nodes, the apparatus parameters of thefocus shifting device have to be taken into consideration. Care istaken, of course, that the distance between the target points and thefrequency with which these target points are provided are selected suchthat they can be effected with the shifting speeds of the focus shiftingdevice. However, this is easy to do for the person skilled in the artwhen adapting the control dataset to a current treatment apparatus.

The method according to the invention for preparing the control data canbe effected without human assistance. In particular, it may be performedby a computer which determines from corresponding data, e.g. from thefunctionally defined path curves, the control data by selecting thenodes so as to match the shifting speed of the laser device representingthe target system for application of the control data. In particular,determining the control data does not require any assistance from aphysician, because said determination of the control data does not yetinvolve a therapeutic intervention. Such intervention will take placeonly when applying the previously determined control data.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail below, by way of exampleand with reference to the Figures, wherein:

FIG. 1 shows a schematic view of a treatment apparatus or instrument forcorrection of defective eyesight,

FIG. 1 a shows a schematic view with respect to the construction of thetreatment instrument of FIG. 1,

FIG. 2 shows a schematic diagram relating to the introduction of pulsedlaser radiation into the eye for the correction of defective eyesight bythe treatment apparatus of FIG. 1,

FIG. 3 shows a further schematic representation of the treatmentapparatus of FIG. 1,

FIG. 4 shows in partial Figures (a), (b) and (c) schematic sectionalviews illustrating the need for correction at the human eye in case ofdefective eyesight,

FIG. 5 shows a schematic sectional view through the eye's cornea andalso represents a volume to be removed for correction of defectiveeyesight,

FIG. 6 shows a section through the eye's cornea after removal of thevolume of FIG. 5,

FIG. 7 shows a sectional view similar to that of FIG. 5;

FIG. 8 shows a schematic sectional view through the eye's corneaillustrating the removal of said volume,

FIG. 9 shows a top view of a spiral-shaped path curve used to isolatethe volume of FIGS. 5, 7 and 8,

FIG. 10 shows an enlarged view of the path curve of FIG. 9,

FIG. 11 shows an alternative path curve for the correction even ofcylindrical eyesight defects,

FIG. 12 shows a schematic representation explaining the function of thecontact glass in the treatment apparatus of FIG. 1,

FIGS. 13 to 15 show schematic representations with respect to theeffects of the contact glass by deformation of the eye's cornea,

FIGS. 16 and 17 show schematic views relating to the approximation ofthe volume of the surface posteriorly limiting the volume in the case ofcorrections even of cylindrical eyesight defects,

FIG. 18 shows a schematic view of a correction of the image fieldcurvature as applied when determining control data for the treatmentapparatus of FIG. 1, and

FIG. 19 shows a schematic representation of the sequence of preparingand carrying out a correction of defective eyesight.

DETAILED DESCRIPTION

FIG. 1 shows a treatment apparatus 1 for an ophthalmic method similar tothose described in EP 1159986 A1 and U.S. Pat. No. 5,549,632. Thetreatment apparatus 1 causes a correction of defective eyesight at aneye 3 of a patient 4 by means of treatment laser radiation 2. Theeyesight defect may include hyperopia, myopia, presbyopia, astigmatism,mixed astigmatism (astigmatism in which hyperopia is present in onedirection and myopia is present in a direction perpendicular to theformer), aspherical aberrations and higher-order aberrations. In thedescribed embodiments, the treatment laser radiation 2 is applied as apulsed laser beam focused into the eye 3. The pulse duration is e.g. inthe femtosecond range, and the laser radiation 2 acts by means ofnon-linear optical effects in the cornea. For example, the laser beamcomprises short laser pulses of 50 to 800 fs (preferably 100-400 fs)with a pulse repetition rate of between 10 and 500 kHz. In the describedexemplary embodiment, the components of the apparatus 1 are controlledby an integrated control unit, which alternatively may be providedseparately, of course.

Prior to using the treatment apparatus, the eyesight defect of the eye 3is measured by one or more measuring devices.

FIG. 1 a schematically shows the treatment apparatus 1. In this variant,it comprises at least two devices or modules. A laser device L emits thelaser beam 2 onto the eye 3. Operation of the laser device L is fullyautomatic, i.e. upon a corresponding start signal, the laser device Lstarts deflection of the laser beam 2 and, thus, generates cut surfaces,which are composed in a manner yet to be described and which isolate avolume in the eye's cornea. The laser device L receives, via controllines not designated in detail, the control data required for operationin advance from a planning device P in the form of a control dataset.Transmission is done prior to operation of the laser device L. Ofcourse, communication may also be wireless. As an alternative to adirect communication, it is also possible to arrange the planning deviceP spatially separate from the laser device L and to provide acorresponding data transmission channel.

The control dataset is preferably transmitted to the treatment apparatus1 and, further, operation of the laser device L is preferably blockeduntil a valid control dataset is present at the laser device L. A validcontrol dataset may be any control dataset which is basically suitablefor use with the laser device L of the treatment apparatus 1. Inaddition, validity may also be made subject to the condition thatfurther tests are passed, e.g. whether additional details stored in thecontrol data set in respect of the treatment apparatus 1, e.g. anapparatus serial number, or in respect of the patient, e.g. a patientidentification number, correspond to other details, e.g. read out at thetreatment apparatus or input separately, as soon as the patient is inthe correct position for operation of the laser device L.

The planning unit P generates the control dataset which is provided tothe laser unit L to perform the surgical operation, said datasetconsisting of measurement data and defective-eyesight data determinedfor the eye to be treated. They are supplied to the planning unit P viaan interface S and, in the exemplary embodiment shown, originate from ameasuring device M, which previously measured the eye of the patient 4.Of course, the measuring unit M may transmit the correspondingmeasurement data and defective-eyesight data to the planning device P inany suitable way.

Data transmissions may be effected by means of memory chips (e.g. by USBor memory stick), magnetic storage devices (e.g. diskettes), by radioconnections (e.g. WLAN, UMTS, Bluetooth) or by wire connections (e.g.USB, Firewire, RS232, CAN Bus, ethernet, etc.). Of course, the sameapplies with respect to the data transmissions between the planningdevice P and the laser device L.

A direct connection of the measuring device M to the treatment apparatus1, with respect to the data transmissions which may be used in onevariant, has the advantage that the use of false measurement anddefective-eyesight data is avoided with maximum certainty. This applies,in particular, if the transfer of the patient from the measuring deviceM or the measuring devices, respectively, to the laser device L iseffected by means of a positioning device (not shown in the Figure)cooperating with the measuring device M or with the laser device L,respectively, such that the respective devices recognize whether thepatient 4 is in the respective position for measurement or forintroduction of laser radiation 2, respectively. Transferring thepatient 4 from the measuring device M to the laser device L maysimultaneously allow also the transmission of measurement ordefective-eyesight data to the treatment apparatus 1.

Preferably, suitable means ensure that the planning device P alwaysgenerates the control dataset assigned to the patient 4 and anyerroneous use of a wrong control dataset for a patient 4 is virtuallyimpossible.

The efforts of the laser beam 2 are schematically indicated in FIG. 2.The treatment laser beam 2 is focused into the cornea 5 of the eye 6 byoptics not designated in detail. This forms in the cornea 5 a focuswhich covers a spot 6 and in which the power density of the laserradiation is so high that, in combination with the pulse duration, anon-linear effect occurs in the eye. For example, each pulse of thepulsed laser radiation 2 may generate an optical breakthrough in theeye's cornea 5, which breakthrough in turn initiates a plasma bubbleschematically indicated in FIG. 2. Thereby, tissue is separated in thecornea 5 by means of this laser pulse. When a plasma bubble forms, theseparation of tissue layers covers an area greater than the spot 6 whichthe focus of the laser radiation 2 covers, although the conditions forgenerating the breakthrough are achieved only in the focus. For eachlaser pulse to generate an optical breakthrough, the energy density,i.e. the fluence of the laser radiation, has to be above a certain pulseduration-dependent threshold value. This interrelationship is known tothe person skilled in the art, for example, from DE 69500997 T2.

Alternatively, a tissue-separating effect by the pulsed laser radiationcan also be produced by emitting several laser radiation pulses withinone region, with the spots 6 of several laser radiation pulsesoverlapping. In this case, several laser radiation pulses will thencooperate to achieve a tissue-separating effect.

However, the type of tissue separation which the treatment apparatus 1employs is of no further relevance to the following description. It isonly essential that pulsed treatment laser radiation 2 is used for thispurpose. For example, use can be made of a treatment apparatus 1 asdescribed in WO 2004/032810 A2. Further, it is essential that amultiplicity of laser pulse foci form a cut surface in the tissue, theshape of said cut surface depending on the pattern in which the laserpulse foci are/will be arranged in the tissue. The pattern determinestarget points for the focus position at which one or more laser pulse(s)is (are) emitted and defines the shape and location of the cut surface.The pattern of the target points is of relevance to the methods andapparatuses explained hereinafter and will be described in more detail.

Now, in order to carry out correction of defective eyesight, material isremoved by means of the pulsed laser radiation from an area within thecornea 5 by separating tissue layers which isolate the material and,thus, enable material removal. The material removal causes a change involume in the cornea, which leads to a change in the optical imagingeffect of the cornea 5. This change is dimensioned precisely such thatthe eyesight defect previously determined is/will be corrected thereby,if possible. In order to isolate the volume to be removed, the focus ofthe laser radiation 2 is directed at target points in the cornea 5,usually in a region located below the epithelium and Bowman's membraneas well as above Decemet's membrane and the endothelium. For thispurpose, the treatment apparatus 1 comprises a mechanism for adjustingthe position of the focus of the laser radiation 2 within the cornea 5.This is schematically shown in FIG. 3.

In FIG. 3, elements of the treatment apparatus 1 are indicated onlyinsofar as they are required for understanding the focus adjustment. Asalready mentioned, the laser radiation 2 is collimated in a focus 7 inthe cornea 5, and the position of the focus 7 in the cornea is shiftedsuch that energy from laser radiation pulses is focused into the tissueof the cornea 5 at several locations to produce the cut surface. Thelaser radiation 2 is provided as pulsed radiation by a laser 8. Anxy-scanner 9, which is realized in one variant by two galvanometermirrors with substantially orthogonal deflections, two-dimensionallydeflects the laser beam coming from the laser 8, so that a deflectedlaser beam 10 is present posterior to the xy-scanner 9. Thus, thexy-scanner 9 causes shifting of the position of the focus 7 in adirection substantially perpendicular to the main direction of incidenceof the laser radiation 2 into the cornea 5. For adjustment of the depthposition, a z scanner 11 is provided in addition to the xy-scanner 9,which z scanner 11 is provided, for example, as an adjustable telescope.The z scanner 11 ensures that the z position of the location of thefocus 7, i.e. its position on the optical axis of incidence, is changed.The z scanner 11 may be arranged preceding or following the xy-scanner9. Thus, the coordinates referred to hereinafter as x, y, z relate tothe shift of the location of the focus 7.

The assignment of the individual coordinates to the spatial directionsis not essential for the operative principle of the treatment apparatus1, but for the sake of simpler description z hereinafter always refersto the coordinate along the optical axis of incidence of the laserradiation 2, and x as well as y designate two mutually orthogonalcoordinates in a plane perpendicular to the direction of incidence ofthe laser beam. The person skilled in the art is certainly aware that athree-dimensional description of the position of the focus 7 in thecornea 5 can also be effected by other coordinate systems; in particularthe coordinate system is not required to be orthogonal. Thus, it is notmandatory that the xy-scanner 9 deflects at mutually orthogonal axes;rather, any scanner may be used which is able to shift the focus 7 in aplane in which the axis of incidence of the optical radiation is notlocated. Thus, non-orthogonal coordinate systems are also possible.

Further, non-straight coordinate systems can also be used to describe orcontrol the location of the focus 7, as will also be explainedhereinafter. Examples of such coordinate systems are sphericalcoordinates as well as cylindrical coordinates.

In order to control the location of the focus 7, the xy-scanner 9 aswell as the z scanner 11, which jointly realize a specific example of athree-dimensional focus shifting device, are controlled by a controldevice 12 via lines not designated in detail. The same applies to thelaser 8. The control device 3 provides a suitably synchronous operationof the laser 8 as well as of the three-dimensional focus shiftingdevice, realized by way of example using the xy-scanner 9 as well as thez scanner 11, so that the location of the focus 7 in the cornea 5 isshifted such that, in the end, a material of a determined volume isisolated, with the subsequent removal of the volume causing a desiredcorrection of defective eyesight.

The control device 12 operates on the basis of predetermined controldata which prescribe the target points for the focus adjustment. Thecontrol data are usually grouped to form a control dataset. According toone embodiment, said control dataset prescribes the coordinates of thetarget points as a pattern, with the sequence of the target points inthe control dataset determining the serial arrangement of the focuslocations and, thus, ultimately determining a path curve (also brieflyreferred to herein as path). In one embodiment, the control datasetcontains the target points as more specific control values for the focuslocation shifting mechanism, e.g. for the xy-scanner 9 and the z scanner11. For preparation of the ophthalmic method, i.e. before the actualsurgical method can be executed, the target points and preferably alsotheir sequence in the pattern are determined. Prior planning of thesurgical operation has to be effected by determining the control datafor the treatment apparatus 1 whose application will then result inoptimal correction of defective eyesight for the patient 4.

First, the volume to be isolated and to be subsequently removed from thecornea 5 has to be determined. As already described with reference toFIG. 1 a, this requires determination of the need for correction. FIG. 4shows, in partial Figures a), b) and c), the optical conditions at theeye 3 of the patient 4. Without a correction of defective eyesight, thesituation shown in partial Figure a) may be present. Together with theeyelens 13, the cornea 5 causes focusing of an object, located atinfinity, in a focus F which is located on the z axis behind the retina14. The imaging effect results, on the one hand, from the eyelens 13,which is relaxed in the non-accommodated eye, as well as, on the otherhand, from the eye's cornea 5, which is defined substantially by ananterior corneal surface 15 as well as by a posterior corneal surface 16and, due to its curvature, also has an imaging effect. The opticaleffect of the cornea 5 is due to the radius of curvature R_(CV) of theanterior corneal surface. Partial Figure a) shows the eyesight defectonly by way of example; in reality, the above-mentioned, more complexeyesight defects may be present. However, the following description alsoapplies to them, but some of the indicated equations may then include anadditional angular dependence, even if it is not explicitly mentioned.

For correction of defective eyesight, an ancillary lens 17 in the formof spectacles is placed in front of the eye 3, in a known manner and asshown in partial Figure b) of FIG. 4, at a distance d_(HS) from thevertex of the cornea 5. The lens 17 of the spectacles is adapted such,in terms of its refractive power B_(BR), that it shifts the far point ofthe entire system, i.e. of the eye with the spectacles, from the focalpoint F to the focal point F*, which is located on the retina 14.

With respect to the nomenclature used in this description, it should benoted that quantities having an asterisk added are quantities obtainedafter a correction. Accordingly, the focus F* is that focus which ispresent after the optical correction achieved by the lens 17 of thespectacles in the partial Figure b) of FIG. 4.

Based on the justified assumption that a change in the thickness of thecornea 5 mainly modifies the radius of curvature of the anterior cornealsurface 5 facing the air, but not the radius of curvature of theposterior corneal surface 16 facing the interior of the eye, the radiusof curvature R_(CV) of the anterior corneal surface 15 is modified byremoving the volume. The cornea 5 reduced by said volume has an imagingeffect modified such that the then corrected focus F* is located on theretina 14. After correction, a modified anterior corneal surface 15* ispresent, and a correction of defective eyesight is achieved even withoutspectacles.

Therefore, to define the pattern of the target points, the curvature ofthe modified anterior corneal surface 15* to be achieved is determined.The starting point for this is the refractive power of the lens 17 ofthe spectacles, because the determination of the correspondingparameters is a standard method in opthalmology. The following formulaholds true for the refractive power B_(BR)(φ) of the lens 17 of thespectacles:B _(BR)(φ)=Sph+Cyl·sin²(φ−θ)  (1)

In this equation, Sph and Cyl designate the correction values to berealized for spherical or astigmatic errors of refraction and θ refersto the position of the cylindrical axis of the cylindrical (astigmatic)eyesight defect, as they are known to the person skilled in the art ofoptometry. Finally, the parameter φ relates to a cylindrical coordinatesystem of the eye and is counted counter-clockwise, looking towards theeye, as is common in opthalmology. Now, using the value B_(BR), thecurvature of the modified anterior corneal surface 15* is set asfollows:R _(CV)*=1/((1/R _(CV))+B _(BR)/((n _(c)−1)(1−d _(HS) ·B_(BR))))+F,  (2)

In equation (2), n_(c) refers to the refractive power of the material ofthe cornea. The corresponding value is usually 1.376; d_(HS) refers tothe distance at which spectacles with a refractive power B_(BR) have tobe located relative to the corneal vertex, so as to produce the desiredcorrection of defective eyesight by means of spectacles; B_(BR) refersto the aforementioned refractive power of the spectacles according toequation (1). The indication of the refractive power B_(BR) may alsocover eyesight defects which go beyond a normal spherical or cylindricalcorrection. B_(BR) (and, thus, automatically also R_(CV)*) will thenshow additional coordinate dependencies.

The factor F expresses the optical effect of the change in the thicknessof the cornea and can be regarded as a constant factor as a firstapproximation. For a high-precision correction, the factor can becalculated according to the following equation:F=(1−1/n _(c))·(d _(C) *−d _(C)),  (3)wherein d_(C) and d_(C)*, respectively, are the corneal thicknessrespectively before and after the optical correction. For a precisedetermination, R_(CV)* is calculated in an iterative manner by deductingthe quantity (d_(C)*−d_(C)) from the difference (R_(CV)*−R_(CV)) duringthe i^(th) calculation and applying the result of the change inthickness obtained in the (i+1)^(th) calculation. This may be continueduntil an abortion criterion is fulfilled, for example when thedifference of the result for the change in thickness is below a suitablydefined limit in two consecutive iteration steps. This limit may bedefined, for example, by a constant difference which corresponds to aprecision of the refractive correction that is adequate for thetreatment.

If the change in thickness of the eye's cornea is neglected, which is,in fact, allowable for a simplified method, F in equation (2) may alsobe set to zero, i.e. neglected and omitted, for a simplifiedcalculation. Surprisingly, this yields the following simple equation forthe refractive power of the modified cornea 5*:B _(CV) *=B _(CV) +B _(BR)/(1−B _(BR) ·d _(HS))

For the person skilled in the art, the equation B_(CV)*=(n−1)/R_(CV)*will easily yield the radius R_(CV)* of the anterior corneal surface 15*which has to be present after the modification so as to obtain thedesired correction of defective eyesight as follows:R_(CV)*=1/((1/R_(CV))+B_(BR)/((n_(c)−1)(1−d_(HS)·B_(BR))))

For the volume, whose removal causes the above change in the curvatureof the anterior corneal surface 15, the boundary surface limiting thevolume is then determined. In doing so, it is preferably to be takeninto consideration that the diameter of the region to be corrected and,thus, the diameter of the volume to be removed should extend, ifpossible, over the size of the pupil in the dark-adapted eye.

In a first variant, a free form surface is defined by numeric methodsknown to the person skilled in the art, which surface circumscribes avolume whose removal causes the change in curvature. For this purpose,the change in thickness along the z axis required for the desiredmodification of the curvature is determined. This will yield the volumeas a function of r and φ (in cylindrical coordinates), which will inturn yield the boundary surface thereof.

A simple analytic calculation is provided by the following, secondvariant, wherein the boundary surface of the volume is built up by twopartial surfaces, namely an anterior partial surface located towards thecorneal surface 15 and a posterior partial surface located opposite. Thecorresponding relationships are shown in FIGS. 5, 6 and 7. The volume 18is limited towards the anterior corneal surface 15 by an anterior cutsurface 19 which is at a constant distance d_(F) below the anteriorcorneal surface 15. With analogy to laser keratomes, this anterior cutsurface 19 is also referred to as a flap surface 19, because there, incombination with a cut opening the eye's cornea 5 towards the edge, itserves to enable lifting of a lamella in the form of a “flap” from theunderlying cornea 5. This type of removal of the previously isolatedvolume 18 is also possible here, of course.

The anterior cut surface 19 has a curvature which is located at d_(F)below the anterior corneal surface 15. If said curvature is spherical, aradius of curvature can be given for the flap surface 19 which is theradius of anterior corneal surface curvature R_(CV) less d_(F). As willbe described later for preferred variants, when generating the cutsurface 19, a contact glass can ensure that the anterior corneal surface15 is spherical at the time the cut surface is generated, so that thepattern of the target points will cause a spherical cut surface.Although the relaxation of the eye 3 after removal of the contact glassmay then lead to a non-spherical cut surface 19, the latter will stillbe at a constant distance from the anterior corneal surface 15 or 15*,respectively. This will be explained later.

The volume 18 to be removed from the cornea 5 is posteriorly limited bya posterior cut surface 20 which, in principle, cannot not be at aconstant distance from the anterior corneal surface 15. Therefore, theposterior cut surface 20 will be provided such that the volume 18 ispresent in the form of a lenticle. For this reason, the posterior cutsurface 20 is also referred to as lenticle surface 20. By way ofexample, said surface is also indicated in FIG. 5 as a spherical surfacehaving a radius of curvature R_(L), the center of said curvature, ofcourse, not coinciding with the center of curvature of the anteriorcorneal surface, which is also spherical in FIG. 5.

FIG. 6 shows the conditions after removal of the volume 18. Now, theradius of the modified anterior corneal surface 15* is R_(CV)* and canbe calculated, for example, according to the previously describedequations. The thickness d_(L) of the removed volume 18 is decisive forthe change in radius, as made clear by FIG. 7. This Figure shows asfurther quantities the height h_(F) of the spherical cap defined by theanterior cut surface 19, the height h_(L) of the spherical cap definedby the posterior cut surface 20 as well as the thickness d_(L) of thevolume 18 to be removed.

Due to the constant distance between the anterior corneal surface andthe anterior cut surface 19, the posterior cut surface determines thecurvature of the anterior corneal surface 15* after removal of thevolume 18. Thus, for example, in the case of a correction of defectiveeyesight taking cylindrical parameters into consideration, the posteriorcut surface 20 will have an angularly dependent radius of curvature. Thefollowing generally applies for the lenticle surface 20 shown in FIG. 7:R _(L)(φ)=R _(CV)*(φ)−d _(F),or in cylindrical coordinates (z, r, φ):z _(L)(r,φ)=R _(L)(φ)−(R _(L) ²(φ)−r ²)^(1/2) +d _(L) +d _(F).

If astigmatism is not to be taken into consideration, the dependence onφ will not occur and the lenticle surface 20 becomes spherical. However,assuming a need for a cylindrical correction of defective eyesight, thelenticle surface 20 usually has different radiuses of curvature ondifferent axes, although these will generally have the same vertex, ofcourse.

This further implies automatically that the theoretical intersectionline between the flap surface 19 and the lenticle surface 20 will not belocated in a plane, i.e. at constant z coordinates, in the case of acylindrical correction. The smallest radius of curvature of the lenticlesurface 20 is at φ=θ+π/2 and the greatest on the axis θ of thecylindrical eyesight defect, of course, i.e. at φ=θ. In the case of acorrection of hyperopia, unlike the representation in FIG. 7, the vertexof the flap surface 19 and the lenticle surface 20 coincide and thelenticle surface 20 has a stronger curvature than the flap surface 19.The thickness d_(L) of the lenticle results as the rim thickness.

The volume 18, which is to be construed as a lenticle, has the smallestrim thickness at φ=θ=π/2, because the lenticle surface 20 and the flapsurface 19 intersect there. For all other values of φ a finite rimthickness is given if a given z coordinate is assumed as the lower limitof the lenticle surface 20.

Alternatively, an additional rim surface can be provided in addition tothe flap surface 20 and the lenticle surface 19, said additional rimsurface circumscribing the volume 18 in the region of intersection ofthe flap surface 20 and the lenticle surface 19 or connecting thesesurfaces, respectively, at points which do not converge at given zcoordinates. Cutting of this rim surface is also effected by the pulsedlaser beam. For example, the rim surface may have a cylindrical shape,an elliptical shape (in a top view) or a conical shape (in a lateralview).

The design of the volume 18 as being limited by an anterior cut surface19 at a constant distance from the anterior corneal surface 15 as wellas by a posterior cut surface 20 is only one variant for limiting thevolume 18. However, it has the advantage that the optical correction issubstantially defined only by one surface (the lenticle surface 20) sothat the analytical description of the other partial surface of theboundary surface is simple.

Further, optimum safety margins are provided with respect to thedistance of the volume to the anterior corneal surface 15 and theposterior corneal surface 16. The residual thickness d_(F) between theanterior cut surface 19 and the anterior corneal surface 15 may be setto a constant value of, for example, 50 to 200 μm. In particular, saidthickness may be selected such that the epithelium, which is sensitiveto pain, remains within the lamella formed by the flap surface 19 belowthe anterior corneal surface 15. Also, the design of the spherical flapsurface 19 is in continuity with previous keratometer cuts, which isadvantageous for the acceptance of the method.

After generating the cut surface 19 and 20, the volume 18 thus isolatedis then removed from the cornea 5. This is schematically represented inFIG. 8, which further shows that the cut surfaces 19 and 20 aregenerated by the influence of the treatment laser beam incident in afocus cone 21, for example by sequential arrangement of plasma bubbles,so that the flap cut surface 19 and the lenticle cut surface 20, in apreferred embodiment, are generated by suitable three-dimensionalshifting of the focus position of the pulsed laser radiation 2.

As an alternative, only the flap surface 19 may be formed, in asimplified embodiment, by target points defining the curved cut surface19 at a constant distance from the anterior corneal surface 15, by meansof pulsed laser radiation, and removal of the volume 18 is effected bylaser ablation, for example by the use of an excimer laser beam. Forthis purpose, the lenticle surface 20 can be defined as the boundarysurface of such removal, although this is not mandatory. In thisrespect, the treatment apparatus 1 works like a known laser keratome;however, the cut surface 19 is produced on curved cornea. The featuresdescribed above and below, respectively, are also possible in suchvariants, especially with respect to the determination of the boundarysurface, its geometric definition and the determination of controlparameters.

If both the lenticle surface 20 and the flap surface 19 are generated bymeans of pulsed laser radiation, it is convenient to generate first thelenticle surface 20 and second the flap surface 19, because the opticalresult in the lenticle surface 20 will be better (or even achieved inthe first place), if no change of the cornea 5 has occurred above thelenticle surface 20 yet.

The removal of the volume 18 by the pulsed laser radiation can beachieved, as indicated in FIG. 8, by a peripheral cut 22, allowing toextract the volume 18 in the direction of an arrow 23 shown in FIG. 8.However, as an alternative, the peripheral cut 22 may be provided suchthat it connects the anterior cut surface 19, i.e. the flap surface 19,with the anterior corneal surface 15, thereby forming a ring, althoughsaid peripheral cut does not extend fully around an angle of 360°. Thelamella isolated in this way remains connected to the residual tissue ofthe cornea 5 in a narrow region. This connecting bridge will then serveas a hinge, so as to be able to fold the otherwise isolated lamella awayfrom the cornea 5 and to remove from the rest of the eye's cornea 5 thealready isolated volume 18 made accessible in this manner. The positionof the connecting bridge can be predetermined when generating thecontrol data or the target points, respectively. Thus, the describedmethod or apparatus, respectively, realizes, according to this aspect,the isolation of the volume 19 within the cornea 5 and generates a flap,connected with the rest of the eye's cornea via a tissue bridge, as alid over the volume. Said lid can be folded away and the volume 18 canbe removed.

To generated the cut surfaces 19 and 20, the target points can easily bearranged in various ways. The prior art, for example WO 2005/011546,discloses special spirals to generate cut surfaces in the eye's cornea,which spirals extend, for example, in the manner of a helical linearound a main axis being substantially perpendicular to the optical axis(z axis). Also, the use of a scanning pattern is known which arrangesthe target points in lines (cf. WO 2005/011545). Of course, theseoptions can be used to generate the above-defined cut surfaces and canbe employed with by the below-explained transformations.

The adjustment of the position of the focus in the eye's cornea iseffected by means of the three-dimensional deflecting deviceschematically shown in FIG. 3, which device employs the shifting oflenses or other optically effective elements to adjust the focus in thez direction. However, the adjustment of lenses or the like is usuallynot feasible as fast as the swiveling of mirrors which are usually usedin the xy scanner. Therefore, the speed of adjustment of the z scannergenerally limits the rate at which the cut surfaces can be generated inthe eye's cornea. In order to generate the cut surfaces 18 and 19 asquickly as possible, the focus is, therefore, in a preferred embodiment,guided along a spiral-shaped path, with one spiral each being located inthe spatially curved cut surface. Thus, while writing the spiral, the zscanner is adjusted such that the arms of the spiral follow thespatially curved cut surface.

By way of example FIG. 9 shows a path curve 24 as a spiral, which is acircular spiral in the representation shown. The radius of the planarspiral shown increases in circular coordinates as the angle of rotationφ increases, so thatr(φ)=φ·d _(T)/(2π)  (4)holds. In this equation, d_(T) is the distance of the spiral arms; it isshown in FIG. 10, which depicts an enlarged detail of FIG. 9. Thedistance of the individual spots 6 onto which the pulsed laser radiationis focused and at which a plasma bubble, for example, is generated by alaser pulse constantly equals d_(S) in the spiral, so that the angularspacing Δφ of the individual spots 6 at which a laser pulse isintroduced into the tissue isΔφ=d _(S) /r

Since the lenticle surface 20, as already mentioned, is usually notnon-spherical, the path curve 24 along which the laser focus is adjustedis an elliptical spiral for which obviously no constant distance of thespiral arms exists. However, along the main axes a and b, a respectivepath distance d_(Tb) as well as d_(Ta) may be defined, as shown in FIG.11.

FIG. 10 shows the spots 6 so as to make clear the position of the focusof the individual laser pulses. Of course, the plasma bubbles actuallyexpand after introduction of the respective laser pulse to such extentthat the cut surface is generated, and the path curve 24 is then nolonger visible in the cut surface.

For preparation of the surgical method, the definition of the pathcurves 24 by which the cut surfaces are generated has to be effectedafter definition of the cut surfaces 19 and 20.

Of course, when determining the path curves 24, it should be noted thatfinally the volume 18 is to be defined for the eye under normalconditions. The cut surfaces 19 and 20 as explained so far relate to thenatural eye. However, it should be taken into account that the treatmentapparatus 1 uses a contact glass 25 for fixation of the eye, whichcontact glass 25 is placed on the anterior corneal surface 15 of theeye's cornea 5, as shown in FIG. 12. The contact glass 25, which isalready the subject of several patent publications (by way of example,reference is made e.g. to WO 2005/048895 A), is of interest for thepresent description of the treatment apparatus 1 or the related methods,respectively, for preparing and/or carrying out the surgical operationonly insofar as it imparts to the anterior corneal surface 15 a definedcurvature, on the one hand, and as it spatially keeps the eye's cornea 5in a predefined position relative to the treatment apparatus 1, on theother hand. With respect to the spherical curvature of the contact glass25, however, the approach described herein differs considerably from theapproach as described, for example, in WO 2003/002008 A, which uses aplanar contact glass pressing the eye's cornea flat.

When the eye is pressed against the contact glass 25 having a sphericalcontact surface, spatial deformation of the eye occurs. Since the corneais usually compressible only tangentially, i.e. does not change itsthickness when pressing is done in this manner, said pressingcorresponds to a transformation of the eye's coordinate system, as shownin FIG. 13, into the coordinate system of the contact glass shown inFIG. 14. This context is known to the person skilled in the art from WO2005/011547 A1, whose disclosure is incorporated herein by fullreference in this respect. In FIGS. 13 and 14, any coordinates providedwith an apostrophe designate the coordinates of quantities relating tothe contact glass 25 or to the contact glass bottom surface 26 facingthe eye.

Moreover, the contact glass has a still further advantage. Due to thepressing onto the spherical contact glass bottom surface 26, theanterior corneal surface 15 becomes automatically spherical. Thus, theanterior cut surface 19 located at a constant distance below thecornea's anterior surface 15 is also spherical due to the pressing ofthe contact glass, which sphericity leads to a considerably simplifiedcontrol. Therefore, completely independently of other features, it ispreferred for the invention to use a contact glass 25 having a sphericalcontact glass bottom surface 26 and to limit the volume by an anteriorcut surface 19 as well as by a posterior cut surface, predeterminingtarget points for the anterior cut surface which form said cut surfaceas a spherical surface at a constant distance d_(F) below the anteriorcorneal surface 15. For the posterior cut surface, target points are/arebeing predetermined which define for the relaxed eye, i.e. after removalof the contact glass, a curvature corresponding to that which is desiredfor the correction of defective eyesight, except for the distance d_(F)from the anterior corneal surface. Analogous considerations apply to themethod for defining the target points or to the method of surgicaloperation.

The representations in FIGS. 13 and 14 show the coordinatetransformation which occurs at the eye when fitting or removing thecontact glass, respectively. They contain both spherical coordinates (R,α, φ) referring to the origin of the curved surface (anterior cornealsurface 15 or contact glass bottom surface 26) and cylindricalcoordinates (r, z, φ) referring to the vertex of the anterior cornealsurface 15 or of the contact glass bottom surface 26, respectively, saidvertex being defined by the point where the optical axis OA intersects.

During the coordinate transformation from the coordinate systemreferring to the eye, as shown in FIG. 13, to the system referring tothe contact glass, according to FIG. 14, the arc length, i.e. α·R, theradial depth (R_(CV)−R) as well as the angle φ remain unchanged. Thus,the transformation of the shapes of the cut surfaces 19 and 20 used asthe basis for the natural eye, i.e. in the coordinate system of FIG. 13,is an important step in the calculation of the control factors for thethree-dimensional focus adjustment device. The calculation is basicallydifferent than for a planar contact glass, wherein e.g. the flap surface19 degenerates to a plane. Substantially only the shape of the cutsurface 20 has to be transformed because the cut surface 19 merely hasto be generated at a constant distance d_(F) from the anterior cornealsurface 15. Thus, in the transformed system, the cut surface 19 is asphere having a radius of curvature R_(F) which is reduced with respectto the curvature radius of the contact glass bottom surface.

The pressing of the cornea 5 of the eye 3 against the spherically curvedcontact glass bottom surface 26 is illustrated in FIG. 15. There, therepresentation on the right-hand side schematically shows the conditionwhen the contact glass bottom surface 26 touches the anterior cornealsurface at the vertex. For a clearer illustration of the geometricrelationships, the anterior corneal surface 15 is schematically drawn asa circle in FIG. 15, although the curvature is spherical only within asmaller circle segment, of course. Pressing the contact glass 25 ontothe cornea 5 causes the transition, symbolized by the arrow 27, to thecondition on the left-hand side of FIG. 15. Removal of the contact glass25 causes a relaxation of the eye 3 in the opposite direction of thearrow 27.

Due to the conditions mentioned above, the coordinates for each point inthe eye's cornea 5 transform from the system shown in FIG. 13 to thesystem of FIG. 14. Now, this is used as a basis for selecting thecontrol factors for the focus adjustment such that the cut surfaces 19and 20 are to be described in the transformed contact glass system,because only then will they have the desired shapes after removal of thecontact glass 26, i.e. after being transformed back to the naturalcoordinate system of the eye. Since contacting of the anterior cornealsurface 15 is usually effected by suction, the aforementionedtransformation will be referred to hereinafter also as suctiontransformation.

Now, in order to cut the flap surface 19, which is spherical asmentioned above, the following speed of adjustment of the z scanner,i.e. the following feed rate in the z direction, is set:v _(Z)(t)=d _(S) ·f _(L) ·d _(T)/(2π·(R _(F) ² −t·d _(S) ·f _(L) ·d_(T)/π))^(1/2),  (5)wherein f_(L) is the frequency of the laser pulses of the laserradiation 2. Equation (5) requires that the z speed v_(Z) can be freelyset and continuously changed.

If it is desired to write a sphere at a speed vz, which is selected froma group of discrete speeds, which is usually the case if the z scanneris driven by a stepping motor, the time dependence of the radialfunction r(t) will be obtained as:r(t)=[d _(S) ·f _(L) ·d _(T) ·t/π−(d _(S) ·f _(L) ·d _(T) ·t)²/(2π·R_(F))²]^(1/2)  (6)as well as, for the angular function,φ(t)=[4π·d _(S) ·f _(L) ·t/d _(T)−(d _(S) ·f _(L) ·t)² /R ²]^(1/2).  (7)

The t² terms under the square root of the radial function as well as ofthe angular function show that no ideal Archimedian spiral is writtenanymore, i.e. the path and spot bubble distances vary in favor of the zspeed being variable only in steps.

If a constant z feed is desired for the focus adjustment, this will notresult in a sphere, as with the speed according to equation (4), but ina paraboloid, and the following holds true:z(r)=[v _(Z)/(d _(s) ·d _(r))][r ² ·π/f _(L)]  (8)As mentioned, in some treatment apparatuses 1, the speed at which the zscanner shifts the focus in the z direction can be adjusted only withina set of discrete speeds. If it is then desired to write a specificparabola by a given speed v_(Z), the product d_(S)·d_(T) has to beselected accordingly, so that the expression in the first squarebrackets of the equation (8) has the desired value. The distance of thepaths, defined by d_(T), as well as the spot distance along the pathdescribed by d_(S), are consequently suitable to vary so as to write aspecific parabola at given v_(Z).

Any of the equations (5), (6)/(7) and (8) can be used to determine thetarget points and, thus, the control of the focus adjustment, in whichcase the corresponding spiral shape/surface shape then has to be used asa basis, of course. Where it is mentioned below that the equations areused for control, this means, in particular, that the target points aredetermined by means of the equations, which can be effected, forexample, by evaluating the functional equations at equidistant points intime. In one variant of the invention, the speed equations are used toensure that the determined target points do not require adjustmentspeeds which cannot even be realized by the focus adjustment device.

For the shapes of the surfaces 19 and 20 mentioned above and determinedas described, a spiral is fitted to the respective surface. The spiralis written under control as described. The calculation of the z speed aswell as of the r- and φ-speeds takes into consideration the surfaceshape of the surface 19 or 20, respectively.

The lenticle surface 20 is spherical if no cylindrical correction is tobe performed. Therefore, one uses control according to equations (4)/(5)or (6)/(7) to generate such sphere. However, as is known, a sphere canalso be approximated by a paraboloid. Therefore, it is envisaged,according to one inventive variant, to approximate one sphere by aparaboloid in a manner known to the person skilled in the art and toperform control according to equation (8).

Due to the suction transformation according to FIG. 15, the geometry ofthe lenticle surface 20 changes. The lenticle surface 20 has to have thecurvature of the corrected anterior corneal surface 15* in thecoordinate system of the contact glass 25. It cannot be related to acenter of curvature which coincides with the center of curvature of thecontact glass. The curvature defined according to the equation (2) is,thus, converted with respect to the suction transformation.

Of course, the radius of curvature defined in equation (2) is a functionof φ. As already mentioned and as common in opthalmology, two radiusesof curvature r_(a) and r_(b) can be given, respectively: one on the axisθ of the cylindrical eyesight defect and one for an axis perpendicularthereto. Computationally, it is particularly favorable to approximate atoroidal curvature, which thus results in the general case, by aparabola such that the lenticle surface 20 is approximated by aparaboloid. This is preferably effected prior to the contact pressuretransformation, but may also be carried out thereafter.

Said approximation is effected by searching a respective parabola forthe two radiuses of curvature which extends both through the vertex ofthe lenticle surface 20 and through a point located, if possible, at therim. FIG. 16 shows the corresponding conditions in the coordinate systemof the eye. This Figure shows the spherical lenticle surface 20 prior tothe contact pressure transformation. In the Figure, the sections throughthe lenticle surface 20, which is toroidal in the generalized case, aresuperimposed on one another along the two half-axes a and b. Thecorresponding curves are referred to as A and B and are circular, with aradius of curvature r_(a) and r_(b), respectively. On each curve, a rimpoint T is described in cylindrical coordinates by the correspondingradius r as well as the height, these parameters referring to the vertexS which is identical for both half-axis sections. Thus, the point T_(a)is characterized by the radius r_(a) as well as the height h_(a). Thisapplies analogously to T_(b).

Now, a parabola is being searched for, which satisfies h=k·r². Theparabola parameters h_(a) obtained thereby for the parabola along thesemimajor axis a as well as k_(b) for the parabola along the semiminoraxis b define the paraboloid, which is then written while shifting thefocal point in the z direction, e.g. by means of a constant z feed (cf.equation (7)) or which is selected from a set of discrete speeds whenselecting the z speed (modification of equation (7)). The sections shownin FIG. 16 of the toroidal lenticle surface 20 along the semiminor axisb as well as the semimajor axis a refer to the representation in thecoordinate system of the eye.

If the approximation has been carried out by parabola equations afterthe contact pressure transformation, the transformed values will appearthere, of course. The explicit calculation of transformed values can beavoided at this point, if the approximation is effected first and theparabola parameters thus found for the contact pressure transformationare subjected to the coordinate system of the contact glass, whereafterthe conditions shown in FIG. 17 are then present.

The specification of the parabola parameters in the coordinate system ofthe eye according to FIG. 16 is as follows:k _(a)=(z(T _(a))−z(S))/r(T _(a))²,  (9)k _(b)=(z(T _(b))−z(S))/r(T _(b))².  (10)

In equations (9) and (10), z(S) refers to the z coordinate of the pointS. Placing the origin of the coordinate system in the vertex, as in theprevious Figures, the z coordinate will be zero. The coordinate z(T_(a))or z(T_(b)), respectively, as well as r(T_(a)) or r(T_(b)),respectively, are the z or r-coordinates, respectively, of thecorresponding point T_(a) or T_(b), respectively, in the cylindricalcoordinate system.

If the parabola parameters k_(a) or k_(b), respectively, are not neededin the coordinate system of the eye according to FIG. 16, but in thecoordinate system of the contact glass according to FIG. 17, the pointsS, T_(a) and T_(a) will then be replaced by the transformed points S′,T_(a)′, T_(b)′ indicated in FIG. 17.

Now, in order to represent the lenticle surface 20 in thepressure-contacted eye 3 by a (then usually elliptical) spiral with semiaxes r_(a)′ and r_(b)′, said spiral is constructed from a circularspiral of radius r₀′ by elongation in the direction φ=θ and compressionin the direction φ=θ+π/2. Due to the simultaneous elongation andcompression, the average path or spot distance, respectively, ismaintained. If compression or elongation were effected only in onedirection, the average distance would change.

The actual radiuses can be calculated as follows from the radius r₀ ofthe circular spiral, which is shown in broken lines in FIG. 17, with thehelp of ellipticity:e′=r _(a) ′/r _(b)′=(k _(B) ′/k _(a)′)^(1/2).

The ellipticity e′ is the ellipticity of the transformed toroidallenticle surface 20. The parameters k_(b)′ as well as k_(a)′ are givenby the equations (11) and (12) for the transformed points S′, T_(a)′ andT_(b)′, respectively. The parabola parameters result from the fact thatthe circular spiral of radius r₀, from which the lenticle surface 20 isconstructed here, is intended to be an arithmetic or geometric averageof the curvatures of the great and small half-axes of the paraboloid,respectively. r₀′=(r_(a)′·r_(b)′)^(1/2) yields k=(k_(a)·k_(b))^(1/2) forthe parabola parameter, as well as the main axes of the ellipse:r_(a)′=r₀′·(e′)^(1/2) and r_(b)′=r₀′·(e′)^(−1/2).

At this point of the determination of the target points, two path curves24 are obtained, which are described by functional equations. Thepattern of the target points is determined by evaluation of thesefunctional equations.

However, it remains to be considered that the focusing of the laser beamin the focus 7 is subject to a focus position error. This focus positionerror is a property of the optical system, i.e. results from the opticalrealization used. It is governed substantially by the optical design.Moreover, due to limited manufacturing accuracy within the allowedtolerances, the focus position error is apparatus-specific. Therefore,it is convenient to determine said error individually for eachapparatus.

Accounting for the focus position error is effected by a usuallynon-linear transformation (referred to hereinafter also as NLtransformation), which makes it impossible to carry out the NLtransformation by modification of the path curve parameters. In apreferred embodiment of the invention, the focus position error isexpressed by a correction table or a correction function. Said functionis derived from gauging the optics of the treatment apparatus 1. Suchgauging can be effected for a type of apparatus or individually for eachapparatus. The correction function can be obtained by interpolation ofthe results of gauging, e.g. by means of polynomials or splines. Thefocus position error is generally rotationally symmetric with respect tothe optical axis. It will then depend only on r and z in cylindricalcoordinates.

The points previously calculated by means of the path curves arepre-distorted in the NL transformation such that they are locatedexactly at the desired location after introduction of the laser spot bythe optical system having said focus position error. Thus, theapplication of the pre-distorted coordinates compensates for the focusposition error appearing in the optical system.

The NL transformation is based on the idea that for each point havingthe coordinates (z, r) a contact glass sphere, displaced by z₀, can befound, on which this point is located. The vertex of said sphere is thenexactly at z₀(z, r)=z−z_(KGL)(r). The effect of the pre-distortion incompensating the focus position error is illustrated in FIG. 18. Itshows the contact glass bottom surface 26, which is spherical in thepresent example, but may also have any other shape. Due to saidpre-distortion, said sphere becomes a transformed contact glass sphere26^. The parameters taking the pre-distortion of the focus positionerror into consideration are symbolized in FIG. 18 by adding a roof-likesymbol “^” to them. In this case, z₀^=z₀ holds true for a transformedaxial point in the cornea. This allows for the fact that, although thefocus position error is rotationally symmetric in most cases, it alsoresults in a shift along the z direction.

For pre-distortion, the calculated path curves are converted toindividual target point coordinates for the spots, which are thenshifted in the correction transformation, i.e. the NL transformation,symbolized by K(r, z) in FIG. 18. If the focus position error isindicated as function K, the coordinate of each target point merely hasto be evaluated by said function to obtain the shift or the transformedcoordinate, respectively.

As a result, a respective set of target points is present for thesurfaces 19 and 20, along which points the focus is guided. Due to theNL transformation and the contact pressure transformation, thecoordinates with respect to the natural, i.e. free, eye are locatedprecisely in the desired anterior and posterior cut surfaces 19, 20.

The coordinates thus obtained for the target points further have to beconverted to control signals for the three-dimensional deflecting unit,e.g. the xy-scanners as well as the z scanner. For this purpose, acorresponding functional relationship or a corresponding characteristicmap is used, which is known for the scanners and has been previouslydetermined, where applicable.

In particular, the response function was previously determined, inparticular, for the xy-scanners, which are realized as galvanometermirrors in the exemplary embodiment. By applying a frequency sweep tothe galvanometer mirrors as well as measuring the actual galvanometermovement, an amplitude response function and a phase response functionwill be obtained. These will be factored in determining the controlsignals.

To simplify control, not every target point results in a respectiveaiming point for the scanners. Instead, the control device 12 usessample points to characterize the path of the scanner. The number ofpoints is reduced considerably thereby. This may be benefited fromalready during the NL transformation, by subjecting to saidtransformation only those target points of the path curves which areintended to be sample points for control. Thus, according to oneembodiment of the invention, evaluation of the functional equations iseffected by means of a time interval which is larger than the timeinterval between the laser pulses.

Thus, prior to the NL transformation, the path curve points are filteredto provide the aforementioned nodes, i.e. points occurring at afrequency of the scanner control, for the transformation. An equivalentto such filtering is an evaluation of the functionally described pathcurves at nodes, which are spaced apart in accordance with the scannercontrol. This reveals a further advantage of the function-based approachdescribed herein: The decision, which points are target points forcontrolling the focus adjustment device, has to be made no sooner thanprior to the NL transformation. Before, only the path parameters have tobe converted in a suitable manner. Also, datasets comprising amultiplicity of points will not occur until then.

Thus, the sample points define target points which are only a subset ofthe set of points to which a laser pulse is emitted. This is illustratedin FIG. 10, in which those spots 6, which exist in the control datasetas target points 28, are shown as solid black circles.

This approach further has the advantage that the maximum frequencyf_(S), which occurs during control of the scanner, is considerablysmaller than the laser pulse frequency f_(p). For example, a controlfrequency of 20 kHz as well as a laser pulse frequency of 200 kHz can beworked with. This has the result that one or more spots 6 onto whichpulsed laser radiation is also emitted are located between the targetpoints 28 provided to control the scanner.

Thus, there is not only an emission of pulsed laser radiation while thescanners are being subjected to an adjustment operation, e.g. while thegalvanometer mirrors are moving, but laser pulses are deflected by thescanners while the latter are moving from one predetermined target pointto the next. In order to achieve a maximum speed of deflection, thismovement represents an oscillation which, in the case of perfectcircular spirals (as they occur in the anterior cut surface 19), is evena purely sinusoidal oscillation. Since, in the case of the lenticlesurface 20, too, the actual spiral shape deviates only slightly fromideal circular or elliptical spirals, the scanners can be operated atnear their maximum frequency, so that the written paths along which thespots are arranged allow very quick production of cut surfaces.

Determining the control datasets which contain the target points fromthe above-described sets of points, which were obtained for the pathcurves, completes the preliminary procedure, which was carried out so asto generate the corresponding control values or control parameters. Thispreliminary procedure requires no human intervention and, in particular,no intervention from a physician or surgeon. The method is carried outby the control device 12 without any action by a physician. Thephysician's presence is not required until the subsequent surgicaloperation.

The procedure of the method for preparing the apparatus 1 for use in anophthalmic operation of defective eyesight is schematically summarizedin FIG. 19. In a step S1, the eye 3 is measured. In doing so, correctionparameters such as those which are common for conventional spectacles,for example, are obtained for the eyesight defect of the patient 4. Theparameters established in step S2 are then used, in a step S3, todetermine the new curvature of the cornea 5 required for the correctedeye. Once this calculation in step S3 has been completed, the volumewhich is to be removed from the cornea is determined in S4. This isusually done by defining the lenticle surface 20 as well as the flapsurface 19 in a step S5. Once the corresponding functional descriptionsof these surfaces have been achieved, the suction transformation, whichis effective in sucking the eye to the contact glass, is taken intoconsideration in step S6.

Next, the coordinates of the path curves, from which the cut surfacesare constructed, are determined. This is schematically indicated in stepS7 by the parameters r, φ, z. At the end of step S7, a point pattern ispresent, comprising the coordinates of the spots on which a laserradiation pulse is to act. The density of the target points can alreadybe reduced to simplify the calculation effort. For this purpose, whencontrolling the scanners, not every spot to which laser radiation isapplied is a sample point.

In the following, the set of coordinates thus obtained is transformedagain in step S8 to allow for the focus position error. Then, in a stepS11, the actual control parameters are determined, including a responsefunction, which was obtained in a step S10 from a previous measurement(step S9) of the amplitude behavior and frequency behavior of thescanners.

Using the thus-determined control parameters, the actual operation isthen performed in step S12, during which operation additional spots,preferably located between the individual locations of support uponwhich the control of the scanners is based, are irradiated with laserradiation pulses.

The invention claimed is:
 1. A method for preparing control data for a treatment apparatus for surgical correction of defective eyesight in a patient's eye, the apparatus comprising a laser device separating corneal tissue of a cornea by irradiation of pulsed laser radiation, said laser device comprising a contact glass that fixates the cornea focusing the laser radiation through the contact glass on target points, which are arranged in a pattern in the cornea, according to the control data during operation, said method comprising: determining measurement data on parameters of the eye and defective-eyesight data on the eyesight defect to be corrected in the eye, defining a volume on the basis of the measurement data and the defective-eyesight data, said volume being located within the cornea and the removal of the volume from the cornea provided after operation of the treatment apparatus resulting in the desired correction of defective-eyesight, determining a boundary surface which confines the defined volume within the cornea, and determining a three-dimensional pattern of target points in the cornea, wherein the target points are located in the boundary surface and are arranged such that the boundary surface is provided as a cut surface when irradiating the pulsed laser irradiation according to the control data, said cut surface isolating the defined volume in the cornea and, thus, making the defined volume removable, wherein, when determining the boundary surface or the three-dimensional pattern of the target points, deformation of the eye that is due to the contact glass is taken into consideration, such that the determined boundary surface is present after operation of the laser device in the undeformed eye, and generating a control data set which contains the three-dimensional pattern for control of the laser device; wherein the defined volume is defined such that the anterior corneal surface of the eye assumes a radius of curvature R_(cv)* after removal of the volume, said radius satisfying the following equation: R _(cv)*=1/((1/R _(cv))+B _(BR)/((n _(c)−1)(1−d _(HS) ·B _(BR))))+F, wherein R_(cv) is the radius of curvature of the cornea prior to removal of the volume, n_(c) is the refractive power of the material of the cornea and F is a correction factor; wherein B_(BR) is the refractive power of spectacles suitable for correction of the defective-eyesight and d_(HS) is a distance at which the spectacles having the refractive power B_(BR) should be located anterior of the corneal vertex to achieve the desired correction of defective-eyesight by the spectacles.
 2. The method of claim 1, wherein the measurement data and the defective-eyesight data are generated by measurement of the eye.
 3. The method of claim 1, wherein the control data set is transmitted to the treatment apparatus.
 4. The method of claim 1, wherein optical focus position errors, which lead to a deviation between the predetermined position and the actual position of the target points when focusing the pulsed laser radiation, are compensated for, when determining the boundary surface or the three-dimensional pattern of the target points, by a pre-offset depending on the position of the respective target point.
 5. The method of claim 4, wherein, in order to determine the pre-offsets, a correction table or correction function is used, which indicates the focus position error depending on the position of the respective target points.
 6. The method of claim 1, wherein the boundary surface comprises an anterior and a posterior partial surface, wherein one of said partial surfaces is and the other one is not located at a constant distance from the anterior corneal surface.
 7. The method of claim 1, wherein the defective-eyesight data comprises B_(BR) as well as the distance d_(HS).
 8. The method of claim 1, wherein F=(1−1/n _(c))·(d _(c) *−d _(c)) holds true, wherein d_(c) is a thickness of the cornea before removal of the volume and d_(c)* is the thickness of the cornea after removal of the volume and the planning module (P) computes the radius R_(cv)* in an iterative manner by deriving the quantity (d_(c)*−d_(c)) from the difference (R_(cv)*−R_(cv)) during each iteration step, and applying the corresponding result for the change in thickness to calculate R_(cv)* in the next iteration step.
 9. The method of claim 6, wherein the partial surface which is not located at a constant distance from the anterior corneal surface is the posterior partial surface, which has a radius of curvature equal to R_(cv)* minus the constant distance between the anterior partial surface and the anterior corneal surface.
 10. The method of claim 1, wherein the control data set is provided for a laser device, which adjusts the focus laser radiation along a path via the target points of the pattern, and that the control data set is generated such that the target point in the pattern represent only a subset of the points onto which the laser device emits the pulsed laser radiation.
 11. The method of claim 10, wherein the control data set is provided for a laser device, which emits the pulses of the pulsed laser radiation into the cornea at a frequency f_(P), and that the control data set contains the pattern of the target point such that, when operating the treatment apparatus, the target points are supplied to the laser device at a frequency f_(S) which is smaller than the frequency f_(P).
 12. A method for generating control data for a laser device (L) of a treatment apparatus (1) for surgical correction of defective eyesight in an eye (3) of a patient (4), which laser device (L) separates corneal tissue by irradiation of focused laser radiation (2), wherein the control data provide the laser device (L) with target points (28) for the focused laser radiation (2) during operation of the treatment apparatus (1), which target points are arranged in a pattern in the cornea (5), such that a volume (18) in the cornea (5) is isolated thereby, the removal of said volume (18) from the cornea (5) causing the desired correction of defective eyesight, wherein the pattern is determined such that the cornea (5) reduced by said volume (18) has a radius of curvature R_(CV)* which satisfies the following equation: R _(CV)*=1/((1/R _(CV))+B _(BR)/((n _(c)−1)(1−d _(HS) ·B _(BR))))+F, wherein R_(CV) is the radius of curvature of the cornea (5) before removal of the volume (18), n_(c) is the refractive power of the material of the cornea (5), F is a correction factor, B_(BR) is the refractive power of spectacles (17) suitable for correction of defective eyesight, and d_(HS) is the distance at which the spectacles (17) having the refractive power B_(BR) should be located anterior of the corneal vertex in order to achieve the desired correction of defective eyesight by means of the spectacles (17).
 13. A method for surgical correction of defective eyesight in an eye (3) of a patient (4), wherein laser radiation (2) is focused on target points (28), arranged in a pattern in the cornea (5), to separate corneal tissue and to thereby isolate and remove a volume in the cornea, said removal causing the desired correction of the defective eyesight, wherein the cornea (5) reduced by said volume (18) assumes a radius of curvature R_(CV)* which satisfies the following equation: R _(CV)*=1/((1/R _(CV))+B _(BR)/((n _(c)−1)(1·d _(HS) ·B _(BR))))+F, wherein R_(CV) is the radius of curvature of the cornea (5) before removal of the volume (18), n_(c) is the refractive power of the material of the cornea (5), F is a correction factor, B_(BR) is the refractive power of spectacles (17) suitable for correction of defective eyesight, and d_(HS) is the distance at which the spectacles (17) having the refractive power B_(BR) should be located anterior of the corneal vertex in order to achieve the desired correction of defective eyesight by means of the spectacles (17).
 14. The method of claim 12 or 13, wherein F=(1−1/n _(c))·(d _(c) *−d _(c)) holds true, d_(c) or d_(c)*, respectively, is the thickness of the cornea (5, 5*) before or after removal of the volume (18), respectively, and the radius R_(CV)* is calculated iteratively by deriving a change in thickness (d_(c)*−d_(c)) from the difference (R_(CV)*−R_(CV)) in each iteration step, and by applying the result thus obtained for the change in thickness, when calculating R_(CV)* in the next iteration step.
 15. The method of claim 12 or 13, wherein, in order to compensate for a cylindrical eyesight defect of the eye (3), the radius R_(CV)* is a function of the cylindrical angle Φ, i.e. R_(CV)*(Φ)=1/((1/R_(CV)(Φ))+B_(BR)(Φ))/((n_(c)−1)(1−d_(HS)·B_(BR)(Φ)))))+F holds true.
 16. The method of claim 12, wherein the pattern confines the volume (18) by a boundary surface, which comprises an anterior and a posterior partial surface (19, 20), wherein the anterior partial surface (19) is located at a constant distance d_(F) from the anterior corneal surface (15), but the posterior partial surface (20) is not.
 17. The method of claim 13, wherein the pattern confines the volume (18) by a boundary surface, which comprises an anterior and a posterior partial surface (19, 20), wherein the anterior partial surface (19) is located at a constant distance d_(F) from the anterior corneal surface (15), but the posterior partial surface (20) is not.
 18. The method of claim 16 or 17, wherein the posterior partial surface (20) is curved and has a radius of curvature R_(L)=R_(CV)*−d_(F).
 19. The method of claim 16, wherein the posterior partial surface (20), in cylindrical coordinates (z, r, φ) whose origin is located at the point where the axis of vision (OA) passes through the anterior corneal surface (15), satisfies the equation Z _(L)(r,Φ)=R _(L)(φ)−(R _(L) ²(φ)−r ²)^(1/2) +d _(L) +d _(F), wherein d_(L) determines a minimum thickness of the volume (18) to be removed.
 20. The method of claim 17, wherein the posterior partial surface (20), in cylindrical coordinates (z, r, φ) whose origin is located at the point where the axis of vision (OA) passes through the anterior corneal surface (15), satisfies the equation Z _(L)(r,Φ)=R _(L)(φ)−(R _(L) ²(φ)−r ²)^(1/2)2+d _(L) +d _(F), wherein d_(L) determines a minimum thickness of the volume (18) to be removed.
 21. The method of claim 2, further comprising using one or more of the following measurement devices: a refractometer, a keratometer, an aberrometer, a wavefront measurement device.
 22. The method of claim 3, further comprising blocking operation of the laser device until a valid control data set is present at the laser device. 